Thiorphan, Tiopronin, and Related Analogs As Substrates and Inhibitors of Peptidylglycine A-Amidating Monooxygenase (PAM)

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FEBS Letters 580 (2006) 521–532

Thiorphan, tiopronin, and related analogs as substrates and inhibitors of peptidylglycine a-amidating monooxygenase (PAM)

Neil R. McIntyre, Edward W. Lowe Jr., Geoﬀrey H. Chew, Terrence C. Owen, David J. Merkler* Department of Chemistry, University of South Florida, 4202 E. Fowler Ave., SCA 400, Tampa, FL 33620-5250, USA

Received 24 October 2005; revised 19 December 2005; accepted 19 December 2005

Available online 28 December 2005

Edited by Judit Ova´di

ing of separate catalytic domains. Peptidyl a-hydroxylating Abstract Peptidyglycine a-amidating monooxygenase is a copper- and zinc-dependent, bifunctional enzyme that catalyzes the monooxygenase (PHM) catalyzes the copper-, O2-, and ascor- cleavage of glycine-extended peptides or N-acylglycines to the bate-dependent hydroxylation of the terminal glycyl a-carbon corresponding amides and glyoxylate. This reaction is a key while peptidylamidoglycolate lyase (PAL) catalyzes the zinc- step in the biosynthesis of bioactive a-amidated peptides and, dependent dealkylation of the a-hydroxylglycine intermediate perhaps, the primary fatty acids amides also. Two clinically use- to the amide product and glyoxylate (Fig. 2) [20,21]. ful N-acylglycines are thiorphan and tiopronin, each with a thiol We report here that thiorphan, tiopronin, and number of moiety attached to the acyl group. We report here that thiorphan structurally related analogs are substrates or inhibitors of and tiopronin are substrates for PAM, exhibiting relatively low PAM. Both thiophan and tiopronin bind to PAM with Kd val- KM,app and VMAX,app values. The low VMAX,app values result, ues 6100 lM, yet show low rates of amidation. The K values most likely, from a decrease in active PAM Æ 2Cu(II) as the d for thiorphan and tiopronin are equal to or lower than the enzyme competes ineffectively with thiorphan and tiopronin for free copper. KM,app values for the glycine-extended peptide substrates for 2005 Published by Elsevier B.V. on behalf of the Federation of PAM, which is surprising for such small, simple compounds. European Biochemical Societies. Generally, small structurally simple PAM substrates like N-benzoylglycine and short-chain N-acylglycines exhibit Keywords: Peptidylglycine a-amidating enzyme; Enzyme KM,app values >1.0 mM. Modeling studies indicate that the inhibition; Tiopronin; Thiorphan relatively strong affinity of tiopronin and thiorphan for PAM is a result of their interaction with copper and that the low rates of amidation observed for these compounds, most likely, results from a decrease in the PAM Æ 2Cu(II) concentration. PAM requires two bound copper atoms for maximum activity. 1. Introduction This study provides new insights not only about the pharmacological activity of thiorphan and tiopronin, but also into the Thiorphan (N-[(R,S)-3-mercapto-2-benzylpropionyl]glycine) development of novel drugs targeted against PAM. and tiopronin (N-(2-mercaptopropionyl)glycine) or N-(2-mercapto-1-oxopropyl)glycine, also known as thiola are two thiol containing drugs. Thiorphan, administered as the prodrug 2. Materials and methods racecadotril (also called acetorphan), is a low nanomolar inhibitor of neutral endopeptidase (NEP or enkephalinase, 2.1. Materials EC, 3.4.24.11) [1] and is used as an antidiarrheal [2,3]. Tiopro- a-Hydroxyhippuric acid, a-methylhydrocinnamic acid, isobutyric nin is a metal chelator and antioxidant used to treat cystinuria anhydride, and oxalyl chloride were from Aldrich, tiopronin, S-pro- pylglutathione, and N-acetylglycine were from Sigma, thiorphan was [4], mercury and copper poisoning [5,6], and rheumatoid from Bachem, N-dansyl-Tyr-Val-Gly was from Fluka BioChemika, arthritis [7]. In addition, tiopronin is cytoprotective during and bovine catalase was from Worthington. Recombinant rat PAM myocardial ischemia and reperfusion [8,9] and may find use was gift from Unigene Laboratories, Inc. All other reagents were of as a nephroprotective agent for the toxicity associated with the highest quality available from commercial suppliers. cisplatin treatment [10]. Both thiorphan and tiopronin are N-substituted glycines 2.2. Synthesis N-[(R,S)-2-Methylhydrocinnamoyl]glycine, N-[(R,S)-2-ethylhydro- (Fig. 1). Work from our laboratory [11–14] and from the cinnamoyl]glycine, N-isobutyrylglycine, and S-methyltiopronin were May group [15] has shown that N-substituted glycines, the prepared by conventional procedures and authenticated by elemental N-acyl- and N-arylglycines, are substrates for peptidylglycine analysis (Atlantic Microlab Inc.) and 1H NMR (Inova 400 spectropho- a-amidating monooxygenase (PAM, EC 1.14.17.3). PAM cat- tometer), CDCl3–DMSO-d6 (95:5) or D2O, chemical shifts ppm (d) alyzes the final reaction in a-amidated peptide biosynthesis relative to TMS or DSS. Full experimental details are provided below. 2.2.1. S-Methyltiopronin (N-(2-methylsulfanyl-1-oxopropyl)gly- [16,17] and recent evidence suggests that the enzyme may also cine). A solution of tiopronin (0.8 gm, 4.9 mmol) and sodium car- have a role in the biosynthesis of oleamide and other fatty acid bonate (0.6 gm, 5.7 mmol) in water (6 mL) was stirred with methyl primary amides [18,19]. PAM is a bifunctional enzyme consist- iodide (0.8 gm, 5.6 mmol) until the organic layer disappeared and carbon dioxide evolution ceased (60 min), then acidified (3 M HCl, 4 mL) and extracted repeatedly with diethyl ether. Crude product (0.65 gm, 75%) recrystallized from ethyl acetate m.p. 109–110 C [Ref. [22], * 1 Corresponding author. Fax: +1 813 974 1733. 112–113 C]. HNMR(D2O): d 1.26 (d, J = 7 Hz, 3H), 1.99 (s, 3H), E-mail address: [email protected] (D.J. Merkler). 3.37 (q, J = 7 Hz, 1H) 3.87 (s, 2H).

0014-5793/$32.00 2005 Published by Elsevier B.V. on behalf of the Federation of European Biochemical Societies. doi:10.1016/j.febslet.2005.12.058 522 N.R. McIntyre et al. / FEBS Letters 580 (2006) 521–532

O O

N COOH N COOH H H SH SH Thiorphan Tiopronin (N-[(R,S)-3-mercapto-2-benzylpropionyl]glycine) (N-(2-mercaptopropionyl)glycine)

Fig. 1. Thiorphan and tiopronin structures. The arrows indicate the bond cleaved by PAM.

PHM PAL

O O OH Glyoxylate O O2 H2O

R N COOH R N COOH R NH2 H 2 Cu(II) H Zn(II) 2 Ascorbate 2 Semidehydroascorbate

Fig. 2. The reaction catalyzed by bifunctional PAM. PAM is a comprised of two distinct catalytic entities: peptidyl a-hydroxylating monooxygenase (PHM) and peptidylamidoglycolate lyase (PAL).

2.2.2. N-[(R,S)-2-Ethylhydrocinnamoyl]glycine (N-(2-benzylbutyryl)- Protein Data Bank [26,27]. The enzyme-bound coppers, CuM and glycine). a-Ethylcinnamic acid [23] (3.5 gm, 20 mmol) in aqueous CuH, were selected within atom type parameters to be Cu(I) for reduced sodium hydroxide (22 mL 1 M) was stirred three days with sodium and Cu(II) for oxidized PHM grids. amalgam (sodium, 1.35 gm, 60 mg-at, in mercury, 100 gm). The filtered Several protein grids were prepared: reduced PHM Æ 2Cu(I) with aqueous phase was acidified and extracted with dichloromethane to bound O2, reduced PHM Æ 2Cu(I) with no bound O2, and oxidized give a-ethylhydrocinnamic acid (3.5 gm, quantitative) as an oil, shown PHM Æ 2Cu(II). Prior to determination of the grid enclosure, species by NMR to be free from unsaturated precursor. This acid (2.6 gm, determined to be redundant for ligand binding were removed (nickel, 15 mmol) reacted with oxalyl chloride (5.6 gm, 45 mmol) for 1 h at water, glycerol and co-crystallized substrate). With no further energy 25 C then 15 min at 60 C. The solvent was then stripped of volatiles minimizations of the co- crystallized protein structure, ProteinPrep and the resulting acid chloride was stirred overnight with a solution of was used to add hydrogens and perform a restrained minimization. glycine (1.7 gm, 23 mmol) and sodium carbonate (4.8 gm, 45 mmol) in The bounding box containing all ligand atoms was increased from water (30 mL). Acidification gave a precipitate of the target N-acylgly- 10 A˚ 3 (default) to 14 A˚ 3. This was done to ensure all residues surround- cine. Recrystallized from diethyl ether (100 mL), 2.29 gm, 65%, m.p. ing both PHM copper binding sites were included within the enclosing 1 107–109 C. H NMR (CDCl3): d 0.85 (t, J = 7 Hz, 3H), 1.41–1.50 box. Substrates underwent energy minimization in Chem3D Ultra 8.0 (m, 1H), 1.56–1.65 (m, 1H), 2.23–2.30 (m, 1H), 2.64 (dd, J1 = 6.5 with MM2 and were then imported into Maestro. Standard Precision J2 = 13.5 Hz, 1H), 2.87 (dd, J1 = 8.5 J2 = 13 Hz, 1H), 3.75 (dd, mode was used for ligand docking investigation with 30–50 poses J1 = 4.5 J2 = 19 Hz,1H), 3.95 (dd, J1 =5 J2 = 19 Hz, 1H), 6.16 (s, per ligand. The top pose for each ligand was selected based on the cor- 1H), 7.01–7.20 (m, 5H). relation of Emodel and Glide score, respectively. Positive controls were 2.2.3. N-[(R,S)-2-Methylhydrocinnamoyl]glycine (N-(2-benzylpro- completed for each experiment through re-docking of the co-crystal- pionyl)glycine). a-Methylhydrocinnamic acid (1.64 gm, 10 mmol) lized substrate. reacted with oxalyl chloride (2.5 gm, 20 mmol) for 1 h at 25 C then 15 min at 60 C. The solvent was stripped of volatiles and the resulting 2.4. Determination of K and V values for N-[(R,S)-2- acid chloride stirred overnight with a solution of glycine (1.5 gm, M,app MAX,app 20 mmol) and sodium carbonate (4.1 gm, 40 mmol) in water (40 mL). methylhydrocinnamoyl]glycine, N-[(R,S)-2-ethylhydro- Acidification gave a crystalline precipitate of the target N-acylglycine cinnamoyl]glycine, L-alanylglycine, N-(isobutyryl)glycine, and (1.55 gm, 70%). Recrystallized from diethyl ether (50 mL) mp 113– S-methyltiopronin 114 C. 1H NMR (CDCl3): d 1.65 (d, J = 7 Hz, 3H), 2.51–2.68 (m, Reactions at 37.0 ± 0.1 C were initiated by the addition of PAM 2H), 3.01 (dd, J1 =7J2 = 14 Hz, 1H), 3.87(dd, J1 = 4.5 J2 = 18.5 Hz, (30–70 lg) into 2.0–3.0 mL of 100 mM MES/NaOH pH 6.0, 30 mM 1H), 4.01 (dd, J1 = 4.5 J2 = 18.5 Hz, 1H), 6.20 (br.s, 1H), 7.15–7.29 NaCl, 1% (v/v) ethanol, 0.001% (v/v) Triton X-100, 1.0 lM Cu(NO3)2, (m, 5H). 5.0 mM sodium ascorbate, and the oxidizable substrate (generally 0.3– 2.2.4. N-Isobutyrylglycine (3-aza-4-oxo-5-methylhexanoic acid). 5.0 KM,app). Initial rates were measured by following the PAM-depen- Isobutyric anhydride (32 gm, 200 mmol) was added in one portion to a dent consumption of O2 using a Yellow Springs Instrument Model 53 stirred mixture of glycine (7.5 gm, 100 mmol) and water (30 mL). The oxygen monitor interfaced with a personal computer using a Dataq mixture was heated, eventually to boiling, then stripped of volatiles Instruments analogue/digital converter (model DI-154RS). VMAX,app under reduced pressure to give a waxy solid which was extracted values were normalized to controls performed at 11.0 mM N-acetylgly- with 10 successive portions of boiling diethyl ether. Partial evaporation cine. Ethanol was added to protect the catalase against ascorbate- of the extracts gave the crystalline acylglycine (6.3 gm, 45%) m.p. 104– mediated inactivation and Triton X-100 was included to prevent 1 106 C. H NMR (CDCl3): d 1.17 (d, J = 7 Hz, 6H), 2.41–2.51 (m, 1H), non-specific absorption of PAM to the sides of the oxygen monitor 3.99 (s, 2H), 6.54 (br.s, 1H). chambers.

2.3. Molecular modeling 2.5. Inhibition of O2 consumption from N-acetylglycine by tiopronin Glide (grid-based ligand docking with energetics) from the FirstDis- Reactions at 37.0 ± 0.1 C were initiated by the addition of PAM covery 3.0 suite (www.schrodinger.com) was used for substrate analysis (36 lg) into 2.5 mL of 100 mM MES/NaOH, pH 6.0, 30 mM NaCl, [24,25]. The coordinates of oxidized and reduced peptidylglycine 5.0 mM sodium ascorbate, 1.0% (v/v) ethanol, 0.001% (v/v) Triton a-hydroxylating monooxygenase (PHM) were obtained from the X-100, 10 lg/ml bovine catalase, 1.0 lM Cu(NO3)2, 2.5–20 mM N.R. McIntyre et al. / FEBS Letters 580 (2006) 521–532 523

N-acetylglycine, and 0–100 lM tiopronin. Initial rates were measured 3. Results and discussion by following the PAM-dependent consumption of O2 using the previ- ously mentioned oxygen monitor apparatus. 3.1. Thiorphan and tiopronin as PAM substrates and inhibitors We routinely monitor PAM catalysis using three methods: (a) 2.6. Inhibition of N-dansyl-Tyr-Val-Gly amidation by thiorphan and the substrate-dependent consumption of O2, (b) the substrate- S-methyltiopronin dependent production of glyoxylate, and (c) the conversion of Reactions at 37.0 ± 0.1 C were initiated by the addition of PAM N N (45–50 ng) into 500 lL of 100 mM MES/NaOH, pH 6.0, 30 mM NaCl, -dansyl-Tyr-Val-Gly to either -dansyl-Tyr-Val-a-hydroxy- 5.0 mM sodium ascorbate, 1.0% (v/v) ethanol, 0.001% (v/v) Triton glycine or N-dansyl-Tyr-Val–NH2 (Fig. 2). Of these three meth- X-100, 10 lg/ml bovine catalase, 1.0 lM Cu(NO3)2, 1.25–20 lM ods, O2 consumption is the least sensitive and, thus, is the most 1 N-dansyl-Try-Val-Gly, and 0–120 lM thiorphan or 0–2500 lM difficult to use when employing substrates with a low VMAX. S-methyltiopronin. At the desired time, an aliquot was removed and Attempts to measure O consumption using either thiorphan added to a vial containing one-fifth volume of 6% (v/v) trifluoroacetic 2 acid to terminate the reaction. The acidified aliquots were assayed for or tiopronin as an oxidizable substrate were unsuccessful, a the percent conversion to N-dansyl-Try-Val–NH2 by reverse phase typical result being shown in Fig. 3. The inhibition of O2 con- HPLC as described by Jones et al. [28]. sumption from N-acetylglycine by thiorphan (Fig. 3) indicated that this N-substituted glycine did bind to PAM, but that it was 2.7. Glyoxylate production from thiorphan, tiopronin, and either a substrate with a low VMAX or an inhibitor. Incubation a-hydroxyhippuric acid of 30 lg/ml PAM with either 200 lM thiorphan or 200 lM To first determine if thiorphan and tiopronin were amidated by tiopronin for 3 h at 37 C yielded glyoxylate from both com- PAM, 36 lg of PAM was added to 1.2 mL of 100 mM MES/NaOH, pounds, 64 ± 7 lM glyoxylate from thiorphan and 43 ± 5 lM pH 6.0, 30 mM NaCl, 1.0% (v/v) ethanol, 0.001% (v/v) Trition glyoxylate from tiopronin. A control containing 200 lM S-pro- X-100, 10 lg/mL catalase, 1.0 lM Cu(NO3)2, 5.0 mM sodium ascorbate, and either 200 lM thiorphan or 200 lM tiopronin. After a 3 h pylglutathione, a known PAM substrate [31], generated incubation at 37 C, the reaction was terminated by the addition 181 ± 1 lM glyoxylate. In each, the amount of glyoxylate pro- 300 lL of 6% (v/v) TFA and duplicate 100 lL aliquots were removed duced is the average ±S.D. for duplicate samples. At 83 lM for glyoxylate analysis. substrate, we measured initial rates of glyoxylate production, Initial rates of glyoxylate formation from thiorphan and tiopronin were determined by adding 30 lg of PAM to a 3.0 mL solution con- 460 ± 8 nmol/min/mg for thiorphan and 84 ± 5 nmol/min/mg taining 100 mM MES/NaOH, pH 6.0, 30 mM NaCl, 1.0% (v/v) etha- for tiopronin. The production of glyoxylate from thiorphan nol, 0.001% (v/v) Trition X-100, 10 lg/mL catalase, 1.0 lM and tiopronin indicates that both are PAM substrates, but Cu(NO3)2, 1.8 mM sodium ascorbate, and either 83 lM thiorphan or exhibit low V values relative to the glycine-extended pep- 83 lM tiopronin. At 5 min intervals from 5 to 25 min., a 500 lL ali- MAX quot was removed, added to a vial containing 100 lL of 6% (v/v) tide and N-acylglycine substrates. TFA to terminate the reaction, and the concentration of glyoxylate Because the VMAX,app values for thiorphan and tiopronin formed measured in the acidified samples. are low, we analyzed their interaction with PAM by standard Initial rates of glyoxylate formation from a-hydroxyhippurate as a steady-state inhibition. Both are mixed-type inhibitors vs. a function of the thiorphan concentration were determined by adding glycine-extended peptide substrate with kinetic constants of 10 lg of PAM to a 850 lL solution containing 100 mM MES/NaOH, pH 6.0, 30 mM NaCl, 1.0% (v/v) ethanol, 0.001% (v/v) Trition X-100, Ki,s =82±11lM and Ki,i = 120 ± 30 for thiorphan (data not 2.0 mM a-hydroxyhippurate, and 0–6.0 mM thiorphan. At 5 min shown) and Ki = 33 ± 5.1 lM and Ki,i = 40 ± 10 for tiopronin intervals from 5 to 25 min, a 150 lL aliquot was removed, added inhibition (Fig. 4A). to a vial containing 30 lL of 6% (v/v) TFA to terminate the reaction, Thiorphan inhibits NEP and other zinc-dependent proteins and the concentration of glyoxylate formed measured in the acidified samples. by binding to the protein-bound zinc [32]. As the PAL-cata- Glyoxylate was determined by the spectrophotometric method of lyzed dealkylation is zinc-dependent, thiorphan was tested as Christman et al. [29] as modified by Katopodis and May [15]. Stan- a potential PAL inhibitor. PAL activity within bifunctional dard curves of [glyoxylate] vs. A520 were constructed in the appropri- PAM was measured through the formation of glyoxylate ate buffers using a glyoxylate solution that had been calibrated by from a-hydroxyhippurate, C6H5–CO—NH–CHOH–COOH fi measuring the glyoxylate-dependent oxidation of NADH (De340 = 6.22 · 103 M1 cm1) as catalyzed by lactate dehydrogenase. C6H5–CONH2 + OCH–COOH, in the absence of ascorbate. Thiorphan was observed to weakly inhibit the PAL-dependent dealkylation of a-hydroxyhippurate with an IC = 3.5 ± 2.8. Analysis of steady-state kinetic data 50 Steady-state kinetic parameters (±S.E.) were obtained by a Kaleida- 0.53 mM. Graphe fit of the initial velocity (v) vs. substrate concentration, ([S]), data to 3.2. Analogs of thiorphan and tiopronin as PAM substrates and v ¼ðV MAX;app½SÞ=ðKM;app þ½SÞ ð1Þ inhibitors The affinity exhibited by PAM for such simple N-substituted where K is the apparent Michaelis constant for the oxidizable sub- M,app glycines as thiorphan and tiopronin is surprising given the strate at fixed [ascorbate] and [O2] concentrations and VMAX,app is the apparent maximum initial velocity at saturating [oxidizable substrate]. available structure-activity data indicates that the substrates Initial rate studies that resulted in competitive inhibition were ana- with KM values <100 lM are larger, being either glycine- lyzed by a SigmaPlot 8.0e fit of the v vs. [S] data as a function of extended peptides or long-chain N-acylglycines [11,15,33].Our the inhibitor concentration to Eq. (2). Initial rate studies that resulted hypothesis was that the thiol or thiolate moiety of thiorphan in mixed, inhibition were analyzed by a SigmaPlot 8.0e fit of the v vs. [S] data as a function of the inhibitor concentration to Eq. (3). and tiopronin is responsible for the unexpected results we have obtained regarding the interaction of these molecules with v ¼ðV MAX;app½AÞ=½KM;appð1 þ½I=Ki;sÞþ½A ð2Þ v ¼ðV MAX;app½AÞ=½KM;appð1 þ½I=Ki;sÞþ½Að1 þ½I=Ki;iÞ ð3Þ 1 Ascorbate is a PAM substrate being oxidized to the ascorbate The IC50 value for the inhibition of a-hydroxyhippurate dealkylation radical, semidehydroascorbate, during catalysis. The substrate being by thiorphan was obtained by analysis of the initial velocity, v, vs. [thi- referred to here is the N-substituted glycine substrate that is ultimately orphan] data as described by Corte´s et al. [30]. converted to the amide and glyoxylate. 524 N.R. McIntyre et al. / FEBS Letters 580 (2006) 521–532

225 A 8 100 µM A 7 -1 200 60 µM 6

175 5 40 µM M) µ

4 2 20 µM

[O ], ( 150 40 µM Thiorphan 3 1/Rate, (µmol/min/mg) 0 µM 125 2 µ B 250 M Thiorphan 1

100 Tiopronin 0123456 Time (minutes) -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 1/[N-Acetylglycine], (mM)-1

Fig. 3. The PAM-dependent consumption of O2 in the presence of thiorphan (A) or N-acetylglycine and thiorphan (B). O consumption 2 B 16 2500 µM at 37 C was initiated by the addition of 30 lg of PAM to a 2.4 mL solution containing 100 mM MES/NaOH, pH 6.0, 30 mM NaCl, 1.0% (v/v) ethanol, 0.001% (v/v) Triton X-100, 10 lg/ml catalase, 5.0 mM 14 µ

sodium ascorbate, and either 18.5 lM thiorphan (A) or 5.0 mM N- -1 1500 M acetylglycine (B). In B, small aliquots of 50 mM thiorphan were added 12 at the times indicated by the arrows to bring the ﬁnal thiorphan concentration to 40 and 250 lM. O2 consumption was measured as described in Section 2. 10

8 750 µM PAM. PAM and the related enzyme, dopamine b-monooxygenase (DbM), are both copper-dependent monooxygenases 6 with 2 bound copper atoms per active site [26,34–36]. The inhi- 1/Rate, (µmol/min/mg) bition of PAM by homocysteine-extended peptides [37] and by 4 captopril ((2S)-1-(3-mercapto-2-methylpropionyl)-L-proline) 0 µM [38] has been attributed to the interaction of a sulfur atom with 2 enzyme-bound copper. Similarly, inhibition of DbM by capto- [S-Methyltiopronin] pril, cysteine, and glutathione has been ascribed to in situ chelation of the enzyme-bound coppers [39,40]. Consistent with 0.0 0.2 0.4 0.6 0.8 1/[N-Dansyl-Tyr-Val-Gly], (µM)-1 our hypothesis, we found that thiorphan and tiopronin analogs lacking a thiol moiety bind to PAM with lower affinity Fig. 4. Inhibition of PAM by tiopronin (A) and S-methyltiopronin (Table 1). Replacement of the thiol with a methyl in thiorphan (B). Initial rates were determined at 37 C as described in Section 2. The points are the experimentally determined initial rates and the lines and tiopronin increases the KM,app 40-fold indicating that the —SH moiety contributes approximately 2 kcal/mol in were drawn using the constants obtained by computer fit to either Eq. (2) or Eq. (3), as appropriate. The error bars represent the S.D. of binding energy relative to –CH3. S-Methyltiopronin was syn- duplicate measurements of the initial rates. thesized to further address the effect of the free-thiol (—SH) moiety on binding affinity. Unlike the mixed-type inhibition displayed with tiopronin, S-2-methylthiopronin was observed presented here, and other reports on the inhibition of PAM to be a competitive inhibitor vs. N-dansyl-Tyr-Val-Gly with and DbM by sulfur-containing ligands [37–40], is that the sul- 2 fur-containing ligands either chelate to active site copper, form- a Ki,s = 190 ± 14 lM(Fig. 4B). ing an E-2Cu(II) Æ inhibitor complex, or remove the active site copper, forming a copper-free-E and separate Cu(II) Æ inhibitor 3.3. The decrease in PAM Æ 2Cu(II) concentration by thiorphan complexes. The copper-free forms of PAM and DbM are inac- and tiopronin tive [41,42]. Experimentally, it is difficult to differentiate be- PAM and DbM both contain two active site copper atoms tween these possibilities. One straightforward experiment that undergo reversible oxidation/reduction during their would be to measure the copper stoichiometry before and after respective catalytic cycles [41,42]. Sulfur-containing ligands treatment with the sulfur-containing inhibitor. Unfortunately, bind strongly to Cu(II). Possible explanations for the results this approach will not work because PAM, as isolated, contains no bound copper [34]. Another possible approach would be to 2 The KM value for a substrate should equal the Ki,s value for that measure spectral changes upon the addition of the sulfur-con- compound if it is employed as a competitive inhibitor against a taining inhibitors to PAM. Such changes will show that the sul- different substrate [45]. The ratio of the KM,app/Ki,s,app for S-2- fur is coordinated to the copper, as has been demonstrated [43], methylthiopronin is 2.4 ± 0.41 and does not match the expected ratio of 1.0. While not far out of agreement with the expected ratio, the but are not likely to provide definitive proof that the reason for the discrepancy is not currently understood. copper Æ inhibitor complex is enzyme-bound. N.R. McIntyre et al. / FEBS Letters 580 (2006) 521–532 525

Table 1 Steady-state kinetic for thiorphan, tiopronin and respective analogsa,b 1 1 Compound KM,app (lM) VMAX,app (lmol/min/mg) (V/K)app (M sec ) 82 ± 11 1.2c 2 · 104 Thiorphan O

C6H5 N COOH H

4100 ± 310 6.4 ± 0.32 2.0 ± 0.18 · 103 N-[(R,S)-2-Methylhydrocinnamoyl]glycine O

C6H5 N COOH H

3000 ± 250 8.3 ± 0.27 3.5 ± 0.31 · 103 N-[(R,S)-2-Ethylhydrocinnamoyl]glycine O

C6H5 N COOH H

33 ± 5.1 0.7c 3 · 104 Tiopronin O

N COOH H SH

2400 ± 230 9.8 ± 0.16 5.1 ± 0.50 · 103 L-Alanylglycine O

N COOH H NH2

1600 ± 70 9.5 ± 0.14 7.4 ± 0.34 · 103 N-(Isobutyryl)glycine O

N COOH H

460 ± 73 1.2 ± 0.36 3.3 ± 1.4 · 103 S-2-Methyltiopronin O

N COOH H S aKinetic constants ± S.E. are from computer ﬁts of the initial rate data to Eq. (1). bThe compounds are grouped into structurally related families. c 2 An approximate VMax for thiorphan and tiopronin was calculated using the equation for substrate inhibition (v = VMAX [S]/[KM +[S]+{[S] /Ki}]) and the experimentally determined value for v at [S]=83lM and KM = Ki,s and Ki = Ki,i.

If the observed inhibition results from the removal of copper centration of the copper Æ inhibitor complex. Sugiura and from PAM, the extent of inhibition at any given concentration Hirayama [44] provide a detailed study of the pH-dependent of tiopronin and thiorphan should match the equilibrium con- interaction of Cu(II) with tiopronin in aqueous solution. These 526 N.R. McIntyre et al. / FEBS Letters 580 (2006) 521–532 data enable us to calculate the equilibrium concentrations of and the calculated ratio [PAM Æ 2Cu(II)]/[PAM]total at different all the forms shown in Fig. 5 under the conditions of our tio- concentrations of thiorphan is high (Table 2). These calcula- pronin inhibition experiments. Because similar data does not tions strongly suggest that inhibition observed for thiorphan exist for thiorphan, we used the pKa and pKassociation values results from the chelation of Cu(II) by thiorphan; thus, pre- shown in Fig. 5 to also model our results for the inhibition venting the formation of active PAM Æ 2Cu(II). In other words, of PAM by thiorphan. PAM competes ineffectively with thiorphan for the binding of At pH 6.0 with a solution containing 1.0 lM Cu(II), 1.1 nM Cu(II) from solution. PAM, and 30–120 lM thiorphan, the predominant thiorphan Similar analysis of the tiopronin inhibition is complicated by form is L1 representing >97% of the [thiorphan]total. The total the differences in experimental conditions. The inhibition con- [Cu(II)]free, available to bind to copper-free PAM, decreases stants for tiopronin inhibition were obtained by measuring the from 220 nM at 30 lM thiorphan to 66 nM at 120 lM thior- dependence of the KM,app and VMAX,app for N-acetylglycine phan (Table 2). Information on the dissociation constants for oxidation on [tiopronin]. N-Acetylglycine oxidation was mon- the binding of Cu(II) to PAM is limited, but the best data indi- itored by O2 consumption, a relatively insensitive assay of cates that the enzyme has high affinity for the binding of the PAM activity requiring a high concentration of PAM. Under first Cu(II), Kd 60 nM, while the second Cu(II) binds with these conditions, [Cu(II)]total was 1.0 lM and [PAM]total was a Kd =60nM [46]. Using the Kd values for Cu(II) binding 0.23 lM and the depletion of Cu(II) by binding to PAM cannot and the total [Cu(II)]free, we calculate that the ratio of be ignored in our calculations. We made the following assump- [PAM Æ 2Cu(II)]/[PAM]total rangesfrom21%at30 lMthiorphan tions in order to calculate the values shown in Table 3, (a) the to 47% at 120 lM thiorphan. The agreement between the exper- first Cu(II) binds with a sufficiently low Kd that the [Cu(II)]free imentally determined ratio of VMAX,inhibitor/VMAX,no inhibitor was reduced from 1.0 to 0.77 lM, (b) that [PAM]copper-free is

N COOH 0 H L O SH

N COO ML pKa = 3.60 H S O Cu(II)

L N COO pK = 6.2 H a SH Cu(II) O

pKa = 8.74 N COO ML O S Cu(II) L2 N COO H S

pKa ~ 15 pKassociation = -7.6

3 N COO L

Fig. 5. Equilibrium species for thiorphan and tiopronin in the presence of Cu(II). The pKa values were from Sugiura and Hirayama [44] and Sigel and Martin [63].

Table 2 a Equilibrium concentrations of the thiorphan species, Cu(II)free,and active PAM Æ 2Cu(II)

2 ½PAM2CuðIIÞ V MAX; inhibitor [L1]total (lM) ½L ðlMÞ½L ðnMÞ [ML1](lM) ½ML ðlMÞ [Cu(II)]free (nM) 1 1 1 ½PAMtotal V MAX; no inhibitor 30 29.6 54 0.48 0.30 220 0.78 0.79 60 59.0 110 0.54 0.34 120 0.66 0.65 90 88.9 160 0.56 0.36 90 0.59 0.56 120 118.8 220 0.57 0.36 70 0.53 0.49 a The ligand, L1, is thiorphan. N.R. McIntyre et al. / FEBS Letters 580 (2006) 521–532 527

Table 3 a Equilibrium concentrations of the tiopronin species, Cu(II)free and active PAMÆ2Cu(II)

2 PAM2CuðIIÞ V MAX; inhibitor [L2]total (lM) [L ](lM) [L ] (nM) [ML2](lM) [ML ](lM) [Cu(II)]free (nM) 2 2 2 ½PAMtotal V MAX; no inhibitor 20 19.5 40 0.26 0.16 180 0.75 0.66 40 3.94 70 0.32 0.20 110 0.64 0.49 60 59.3 110 0.34 0.22 80 0.57 0.39 100 99.2 180 0.38 0.24 50 0.46 0.58 a The ligand, L2, is tiopronin.

0, and (c) the Kd of 60 nM defines the equilibrium between reduced PHM with two bound Cu(I) atoms and no bound O2 inactive PAM Æ Cu(II) and active PAM Æ 2Cu(II). While the (GRID2), and oxidized PHM with two bound Cu(II) atoms agreement between the ratio of VMAX,inhibitor/VMAX,no inhibitor (GRID3). The protonated thiol forms of tiopronin and thior- and the calculated ratio [PAM Æ 2Cu(II)]/[PAM]total as a func- phan (L in Fig. 5) and all the analogs included in Table 1 bind tion of [tiopronin] is not as strong as that seen in Table 2 for similarly to reduced PHM, with and without bound O2 thiorphan, the general trend in the data is correct. Again, the (GRID1 and GRID2). This means that the ligand orientation calculations for tiopronin are consistent with our hypothesis within the active site of reduced PHM, as defined by the Emo- that the inhibition caused by the thiol-containing N-substituted del and Glide scores 3, are similar for all the compounds in- glycines is due to the chelation of copper by tiopronin prevent- cluded in Table 1 (Table 4). The thiolate forms of tiopronin ing the formation of active PAM Æ 2Cu(II). The relatively low and thiorphan (L2 in Fig. 5) also bind in a similar orientation, VMAX,app values observed for tiopronin and thiorphan as sub- within GRID1 and GRID2, differing only by a reduction in strates may result from the relatively low levels of active the Cu(I)M–S bond distances (Table 4). PAM Æ 2Cu(II) that exist in the presence of these copper-chelat- While the concentrations of L2 and L3 (the tiopronin and ing substrates. thiorphan forms with an unprotonated carboxylate, thiolate, The mixed-type inhibition pattern obtained for tiopronin and amide nitrogen, see Fig. 5) are low, 0.2% for L2 and and thiorphan provides additional evidence that inhibition 2 · 108% for L3 of the total [ligand] at pH 6.0, we can mediated by these compounds results from the extraction of model the interaction of these forms with PHM. We found a PAM-bound copper. Both are substrates and, thus, must di- significant decrease in the Emodel and Glide scores for the rectly compete for the same enzyme form as the oxidizable sub- binding of the L2 and L3 forms of tiopronin and thiorphan strate, N-dansyl-Tyr-Val-Gly and N-acetylglycine, generating to the oxidized enzyme (GRID3 compared to scores obtained the observed increases in the KM,app as a function of inhibitor. for GRID1 and GRID2) (Table 4). The decreases in both the The concentration dependent removal of copper, decreasing the Emodel and Glide scores are indicative of a higher affinity of concentration of active PAM Æ 2Cu(II), would yield an ob- the enzyme for the docked ligands. Both the L2 and L3 served decrease in VMAX,app as a function of inhibitor. Meth- forms of tiopronin and thiorphan bind strongly to Cu(II)H. ylation of the thiol would significantly reduce the chelation The Emodel and Glides scores for tiopronin and thiorphan de- potential of tiopronin yielding an N-substituted glycine that crease for L3 relative to L2 directly relating to the increased we would expect to be a substrate for PAM that is competitive coordination of tiopronin and thiorphan from the bi-dentate vs. N-dansyl-Tyr-Val-Gly, exactly what we obtain for S-meth- metal Æ ligand complex (ML in Fig. 5) to the tri-dentate metal- ylthiopronin (Table 1 and Fig. 4B). Note that other metal-che- ligand complex (ML in Fig. 5). Models for oxidized PHM lating inhibitors have also been shown to be non-competitive with the L3 forms of tiopronin and thiopronin bound at inhibitors against other metalloenzymes [47–49]. Cu(II)H are shown in Fig. 6A and B. The high affinity interaction between the L2 and L3 forms of tiopronin and thior- 3.4. Modeling of thiorphan, tiopronin, and related analogs in the phan with oxidized PHM compared with both GRID 1 and PHM active site 2 suggest that the presence of oxygen with the PHM grid does The X-ray structures of oxidized and reduced PHM, the cat- not affect the binding affinity of tiopronin and thiorphan, alytic unit catalyzing the copper-dependent hydroxylation of though the oxidation state of the PHM-bound copper appears glycine (Fig. 2), reveal that the two bound copper atoms are to have a large effect on substrate binding affinity. This pro- separated by 11 A˚ across a solvent-accessible active site cleft vides additional evidence for the coordination of PHM-bound 2 3 [26,27]. The catalytic roles of the two coppers are different; CuH(II) by the L and L states of thiorphan and tiopronin CuH is involved in electron transfer while CuM is involved in beyond the chelation of non-enzymatically bound copper. O2 binding and glycine hydroxylation [21]. The PHM-bound coppers being named for their respective ligands, CuH having 3.5. Biological relevance three His ligands and CuM, two His and one Met ligand [50]. Clinical dosing of acetorphan, the prodrug of thiorphan [2], A recent structure of PHM/ce:inf> in a pre-catalytic complex at 300 mg leads to a peak plasma concentration of 0.8–1.0 lM [27],O2 coordinated to CuM and N-acetyldiiodotyrosyl-D-thre- after 1 h, with an elimination half life of 3 h. Patients suffering onine (a substrate analog) in the active site, provides us with an from cystinuria are given daily doses of 1200 mg of tiopronin excellent opportunity to model the sulfur-containing N-substi- [51]. A 500 mg oral dose of tiopronin yields a maximal plasma tuted glycines in the PHM active site to better understand the relatively high affinity observed for these compounds. These 3 compounds must bind in the PHM active site because all are Glide score is a measure of the enzyme–substrate interaction energy (binding affinity) calculated from a molecular mechanics force field. substrates for the enzyme (Table 1). Emodel is a composite scoring function, consisting of the Glide score Three PHM grids were chosen as discussed in the methods: calculation and the internal-strain energy of the docked substrate, used reduced PHM with two Cu(I) atoms and O2 bound (GRID1), to both select and rank enzyme-bound substrate. 528

Table 4 Bond distances in the models of the tiopronin, thiorphan, and related analogs in the PHM active sitea ˚ ˚ ˚ ˚ ˚ ˚ ˚ Compound S–CuM (A)S–CuM (A)S–CuH (A)Ha–CuM (A)Ha–O (A)COO–CuH (A)COO–CuM (A) Emodel score Glide score

(A) GRID1 – reduced PHM with O2 bond, PHM Æ 2Cu(I) Æ O2 Thiorphan (L form, –SH and –NH) 4.4 N/A N.A 5.9 4.9 2.4 5.5 73 7.0 Thiorphan (L2 form, –S and –NH) N/A 6.2 2.6 2.8 3.0 2.3 5.8 88 4.6 Thiorphan (L3 form, –S and –N) N/A 3.6 9.8 4.8 3.9 2.9 5.3 99 6.0 Tiopronin (L form, –SH and –NH) 5.8 N/A N/A 3.0 3.1 2.3 5.8 77 6.6 Tiopronin (L2 form, –S and –NH) N/A 3.1 8.5 5.1 5.6 2.2 5.8 60 5.5 Tiopronin (L3 form, –S and –N) N/A 2.9 8.4 3.5 3.5 2.4 5.5 92 5.1 S-Methyltiopronin(–SCH3 and –NH) 5.6 N/A N/A 5.5 5.4 2.3 5.8 65 7.1 N-(R,S)-2-Methylhydrocinnamoyl)glycine N/A N/A N/A 6.1 5.7 2.3 5.8 69 5.9 N-(R,S)-2-Ethylhydrocinnamoyl)glycine N/A N/A N/A 6.1 5.7 2.2 5.8 70 7.4 L-Alanylglycine N/A N/A N/A 6.0 3.6 2.3 5.8 63 6.8 N-(Isobutyryl)glycine N/A N/A N/A 6.0 3.6 2.3 5.7 59 4.9

(B) GRID2 - Reduced PHM (no bound O2), PHM Æ 2Cu(I) Thiorphan (L form, –SH and –NH) 4.9 N/A N.A 3.9 N/A 5.7 2.3 63 6.1 Thiorphan (L2 form, –S and –NH) N/A 2.6 7.6 5.1 N/A 2.2 5.8 90 6.5 Thiorphan (L3 form, –S and –N) N/A 5.3 7.9 3.0 N/A 5.6 2.3 110 6.1 Tiopronin(L form, –SH and –NH) 5.5 N/A N/A 2.9 N/A 2.3 5.7 60 5.2 Tiopronin (L2 form, –S and –NH) N/A 2.6 7.3 5.4 N/A 2.3 7.0 77 5.3 Tiopronin (L3 form, –S and –N) N/A 4.9 7.4 3.4 N/A 2.4 5.5 95 4.2 S-Methyltiopronin(-SCH3 and –NH) 5.5 N/A N/A 2.3 N/A 2.3 5.7 64 5.5 N-((R,S)-2-Methylhydrocinnamoyl)glycine N/A N/A N/A 4.9 3.6 2.3 2.3 64 6.7 N-((R,S)-2-Ethylhydrocinnamoyl)glycine N/A N/A N/A 5.3 3.8 2.5 2.5 62 6.2 L-Alanylglycine N/A N/A N/A 4.7 5.8 2.3 2.3 65 7.0 N-(Isobutyryl)glycine N/A N/A N/A 5.0 5.7 2.4 2.4 56 5.6 ˚ ˚ ˚ ˚ ˚ ˚ ˚ ˚ 521–532 (2006) 580 Letters FEBS / al. et McIntyre N.R. Compound S–CuM (A)S–CuM (A)S–CuH (A)Ha–CuM (A)Ha–O(A)COO–CuH (A)COO–CuM (A)N–CuH (A) Emodel score Glide score b (C) GRID3, Oxidized PHM(no boundO2), PHM Æ 2Cu(II) Thiorphan (L form, –SH and –NH) 5.5 N/A N.A 4.1 N/A 2.1 7.1 N/A 110 7.5 Thiorphan (L2 form, –S and –NH) N/A 2.5 9.1 5.8 N/A 2.2 8.4 N/A 160 7.3 Thiorphan (L3 form, –S and –N) N/A 9.8 2.5 6.2 N/A 2.1 9.2 2.2 240 8.9 Tiopronin (L form, –SH and –NH) 5.7 N/A N/A 9.5 N/A 2.1 8.7 N/A 100 7.1 Tiopronin (L2 form, –S and –NH) N/A 8.5 2.5 7.5 N/A 2.1 9.2 N/A 150 7.2 Tiopronin (L3 form, –S and –N) N/A 9.4 2.6 7.9 N/A 2.3 9.8 2.3 220 8.5 S-Methyltiopronin(–SCH3 and –NH) 7.2 N/A N/A 4.0 N/A 2.1 9.2 N/A 110 7.7 N-(R,S)-2-Methylhydrocinnamoyl)-glycine N/A N/A N/A 4.1 N/A 2.1 9.0 N/A 110 7.4 N-(R,S)-2-Ethylhydrocinnamoyl)-glycine N/A N/A N/A 9.4 N/A 2.1 9.3 N/A 110 8.2 L-Alanylglycine N/A N/A N/A 9.2 N/A 4.3 10.1 N/A 110 7.7 N-(Isobutyryl)glycine N/A N/A N/A 9.3 N/A 2.1 9.2 N/A 110 7.4 aN/A, not applicable. b 3 The distance from N to CuH is only included in Table 4C because the L forms of thiorphan and tiopronin whould only coordinate to CuH(II) and not CuH(I). Oxidized Cu(II) is only found in GRID3. N.R. McIntyre et al. / FEBS Letters 580 (2006) 521–532 529

3 Fig. 6. Interaction of the PHM CuH(II) binding pocket with the L form of thiorphan (A) and tiopronin (B). The oxidized PHM Æ ligand models, active site binding affinity, and bond distances for thiorphan (A) and tiopronin (B) (ball and stick) to were calculated using Glide (Table 4). Thiorphan and tiopronin are represented as ball and stick models; oxidized PHM as a stick model of the CuH(II) binding pocket containing residues His-106, His-107, and His-172; and red, oxygen; blue, nitrogen; yellow, sulfur; gray, carbon; and green, CuH(II). concentration of 31 ± 10 nM after 4.8 ± 0.9 h with bi-exponen- tiopronin co-localizes with PAM in the blood and the heart tial elimination half lives of 2.2 ± 0.4 and 22.6 ± 11.3 h, respec- only [53,54,56]. Though, being negatively charged at physio- tively [52]. Interestingly, PAM has been found to co-localize logical pH (L in Fig. 5), tiopronin is unlikely to cross the with thiorphan in the blood, heart, and brain [1,53–55] while blood–brain barrier to any significant extent. The significance 530 N.R. McIntyre et al. / FEBS Letters 580 (2006) 521–532 of PAM and thiorphan or tiopronin being found together in Research, the Shin Foundation for Medical Research, and the Shirley the blood and the heart is unclear because the function of W. & William L. Griffin Foundation to D.J.M, by a grant from the Ca- PAM in these tissues is presently not well understood. The mille and Henry Dreyfus Foundation for Senior Scientist Mentor Ini- tiative Award (SI-02-006) to T.C.O., and a predoctoral fellowship to most intriguing site of co-localization is the brain. E.W.L. from the American Heart Association (#0415259B). Analgesic studies with mice indicate that the benzyl ester of acetorphan is readily cleaved in the serum, leaving the metabolite S-acetylthiorphan [1,55]. Lambert et al. [55] suggest this Appendix A. Supplementary data metabolite retains sufficient lipophilicity to cross the blood– brain barrier, distributing thiorphan after thioester hydrolysis Supplementary data associated with this article can be found, within the brain. With the wide distribution of PAM within the in the online version, at doi:10.1016/j.febslet.2005.12.058. brain along with its important regulatory role in hormone acti- vation [21], interaction of an acetorphan metabolite with PAM seems plausible. Our results suggest that thiorphan inhibits References peptide processing in the brain at two steps, the amidation reaction catalyzed by PAM and peptide degradation catalyzed [1] Lecomte, J.-M., Costentin, J., Vlaiculescu, P.C., Marcais-Col- by NEP. lando, H., Llorens-Cortes, M.L. and Schwartz, J.-C. (1986) The interaction of PAM with thiorphan and tiopronin ap- Pharmacological properties of acetorphan, a parenterally active pear to greatly differ. While both compounds interacted with ‘‘enkephalinase’’ inhibitor. J. Pharmacol. Exp. Ther. 237, 937– 944. PAM in vitro (Table 1), the strongest evidence for the potential [2] Matheson, A.J. and Noble, S. (2000) Racecadotril. Drugs 59, inhibition of in vivo amidation appears to be with thiorphan. 829–835. Though, the phenotype of this interaction would, theoretically, [3] Huijghebaert, S., Awouters, F. and Tytgat, G.N.J. (2003) be subtle. NEP inhibition would decrease the proteolysis and Racecadotril versus loperamide. Dig. Dis. Sci. 48, 239–245. [4] Joly, D., Rieu, P., Me´jean, A., Gagnadoux, M.-F., Daudon, M. hydrolysis of opioid and non-opioid peptides [57]. As amida- and Jungers, P. (1999) Treatment of cystinuria. Pediatr. Nephrol. tion has been observed with several enkephalins, thiorphan 13, 945–950. inhibition of PAM would be expected to result in the reduction [5] Formigli, L.M., Ferrari, I. and Grisolia, C.K. (2002) Evaluation of amidated enkephalins, proportional to thiorphan absorption of genotoxic and cytotoxic potential of thiola (N-2-merca- [58]. Therefore, the interplay of both NEP and PAM inhibition ptopropionlyglycine), a medicine used in the treatment of humans contaminated with mercury. Environ. Mol. Mutagen. have the potential to disrupt both primary and tertiary meta- 39, 18–21. bolic pathways associated with enkephalin processing in vivo. [6] Miki, H. (1964) Effect of a-mercapto-propionyl-glycine on urinary excretion of copper in Wilson’s disease. J. Clin. Pediatr. Sapporo 12, 111–115. [7] Pasero, G., Pellegrini, P., Ambanelli, U., Ciompi, M.L., Cola- mussi, V., Ferraccioli, G., Barbieri, P., Mazzoni, M.R., Menegale, 4. Conclusion G. and Trippi, D. (1982) Controlled multicenter trail of tiopronin and D-penicillamine for rheumatoid arthritis. Arthritis Rheum. In conclusion, we have demonstrated that two sulfur-con- 25, 923–929. [8] Horwitz, L.D., Fennessey, P.V., Shikes, R.H. and Kong, Y. taining drugs, tiopronin and thiorphan, inhibit PAM with Kd values <100 lM. These results are of pharmalogical signifi- (1994) Marked reduction in myocardial infarct size due to prolonged infusion of an antioxidant during reperfusion. Circu- cance given the importance of PAM in the biosynthesis of a- lation 89, 1792–1801. amidated peptide hormones and a potential role in the [9] Date, M., Morita, T., Yamashita, N., Nishida, K., Yamaguchi, in vivo production of oleamide and other bioactive fatty acid O., Higuchi, Y., Hirotani, S., Matsumura, Y., Hori, M., Tada, M. primary amides. The inhibition observed for tiopronin and thi- and Otsu, K. (2002) The antioxidant N-2-mercaptopropionyl glycine attenuates left ventricular hypertrophy in in vivo murine orphan most likely results from the decrease in PAM Æ 2Cu(II) pressure-overload model. J. Am. Coll. Cardiol. 39, 907–912. concentration and the coordination of CuH(II), which is essen- [10] Zhang, J.G., Viale, M., Esposito, M. and Lindup, W.E. (1999) tial for catalysis. These results are valuable as a PAM specific Tiopronin protects against the nephrotoxicity of cisplatin in the inhibitor could prove therapeutically useful for the treatment rat. Hum. Exp. Toxicol. 18, 713–717. of small cell lung cancer or prostate cancer. These cancers [11] Wilcox, B.J., Ritenour-Rodgers, K.J., Asser, A.S., Baumgart, L.E., Baumgart, M.A., Boger, D.L., DeBlassio, J.L., deLong, over-express both PAM and growth stimulating a-amidated M.A., Glufke, U., Henz, M.E., King III, L., Merkler, K.A., peptide hormones [59–62]. The most potent PAM inhibitors Patterson, J.E., Robleski, J.J., Vederas, J.C. and Merkler, D.J. developed to date, N-acylated dipeptides with a C-terminal (1999) N-Acylglycine amidation: implications for the biosynthesis homocysteine (IC values as low as 8 nM) contain a thiol moi- of fatty acid primary amides. Biochemistry 38, 3235–3245. 50 [12] Merkler, D.J., Glufe, U., Ritenour-Rodgers, K.J., Baumgart, ety, which is proposed to interact with the one of the PAM- L.E., DeBlassio, J.L., Merkler, K.A. and Vederas, J.C. (1999) bound copper atoms [37]. The work we report here strongly Formation of nicotinamide from nicotinuric acid by peptidylgly- indicates that the future development of PAM inhibitors of cine a -amidating monooxygenase (PAM): a possible alternative route from nicotinic acid (niacin) to NADP in mammals. J. Am. even higher affinity, Kd values <8 nM, should judiciously incorporate a thiol to chelate (and possibly remove) the en- Chem. Soc. 121, 4904–4905. [13] King III, L., Barnes, S., Glufke, U., Henz, M.E., Kirk, M., zyme-bound copper atoms. Merkler, K.A., Vederas, J.C., Wilcox, B.J. and Merkler, D.J. (2000) The enzymatic formation of novel bile acid primary Acknowledgments: This work was supported, in part, by grants from amides. Arch. Biochem. Biophys. 374, 107–117. the National Institutes of Health – General Medical Sciences (R15- [14] DeBlassio, J.L., deLong, M.A., Glufke, U., Kulathila, R., GM067257 and R15-GM073659), the Gustavus and Louise Pfieffer Merkler, K.A., Vederas, J.C. and Merkler, D.J. (2000) The Research Foundation, the University of South Florida – Established amidation of salicyluric acid and gentisuric acid. A possible role Researcher Grant Program, the Wendy Will Case Cancer Fund, the for peptidylglycine a-amidating monooxygenase (PAM) in the Eppley Foundation for Research, the Milheim Foundation for Cancer metabolism of aspirin. Arch. Biochem. Biophys. 383, 46–55. N.R. 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