Identification of Cysteine As the Reactive Group in Pyruvate Kinase Alkylated by 5-Chloro4.Oxopentanoic Acid by R
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Biochem. J. (1976) 159,213-219 213 Printed in Great Britain Identification of Cysteine as the Reactive Group in Pyruvate Kinase Alkylated by 5-Chloro4.oxopentanoic Acid By R. ALAN CHALKLEY and DAVID P. BLOXHAM Department ofPhysiology and Biochemistry, University of Southampton, Southampton S09 3TU, U.K. (Received 8 March 1976) 4-Hydroxypentanoic acid alanine thioether was synthesized and characterized by n.m.r. spectroscopy. This derivative corresponded to the modified amino acid obtained by allowing 5-chloro4oxo[3,5-3H]pentanoic acid to react with rabbit muscle pyruvate kinase. Performic acid oxidation of 4-oxo[3,5-3H]pentanoic acid alanine thioether in pyruvate kinase gave [3H]succinate (67%) and [3H]carboxymethylcysteine (33%) as expected. Evidence is presented to show that NaBH4 reduction followed by periodate oxidation and analysis of radioactive formaldehyde production may provide a con- venient method for distinguishing between thiol and amino alkylation by halogeno- methyl ketone compounds. Peptide 'mapping' confirms that the modification by 5-chloro- 4-oxopentanoic acid occurs primarily at one region of pyruvate kinase. In the preceding paper (Bloxham & Chalkley, (Bloxham & Chalkley, 1976). In all these experiments 1976), 5-chloro-4-oxopentanoic acid was shown to the reaction was terminated by adjusting the pH to specifically alkylate rabbit muscle pyruvate kinase at 7.0 with 1 M-acetic acid before 70% loss of activity to a group which might be located close to the 'phos- eliminate any problems due to protein aggregation. phoryl overlap' region of the active site (Reynard et For the experiments involving amino acid analysis or al., 1961). The next stage in the study of this reaction periodate oxidation the protein was reduced with was to identify the nature of the functional group at 5mg of NaBH4. All the 5-chloro-4-oxopentanoate- the active site. The report describes the synthesis of a labelled pyruvate kinase samples were exhaustively standard reference amino acid (4-hydroxypentanoic dialysed against water at 4°C and finally freeze-dried. acid alanine thioether, I) and the use of performic acid oxidation to analyse the labelled amino acid. Degradation by performic acid oxidation should be Chromatographic analysis ofmodified amino acids particularly useful in this system, since the proposed structure of the inhibitor amino acid adduct Labelled pyruvate kinase (25mg) was hydrolysed in at 0 lOml of6M-HCI 110°C for 18h. The hydrolysate 11 was flash-evaporated and then vacuum-dried twice ml water. was (X-CH2--C-CH2--CH2--CO2H) means that the from 5 of High-voltage electrophoresis compound should be split into two chemically performed with either a Gilson high-voltage electro- identifiable intermediates (see below for detailed phorator model D or Locarte electrophoresis explanation). Although performic acid oxidation has apparatus. The buffers used were pH 1.9 (acetic acid/ been used previously to produce carboxymethyl- formic acid/water; 27:25:888, by vol.), pH3.5 amino acids from more complex amino acids (Petra (pyridine/acetic acid/water; 1:10:289, byvol.), pH4.7 et al., 1965; Husain & Lowe, 1968), the structure of (pyridine/acetic acid/water; 1:1:48, by vol.), and the degraded derivative allowed only the carboxy- pH6.5 (pyridine/aceticacid/water; 25:1:475, byvol.). methyl-amino acid to be analysed, and this meant Amino acids were located as either ninhydrin- that complete information on the mechanism of this positive spots or by radioactivity. For initial location reaction for protein-bound intermediates was not of radioactive spots a Camlab radiochromatogram obtained. The present communication shows that scanner was used. Subsequently the paper was cut there is a high degree of preference in the cleavage into 1 cm strips, soaked in 1 ml of water for 1 h and reaction. Finally, it is suggested that periodate then counted for radioactivity. Amino acid analyses oxidation may be advantageous in the preliminary of hydrolysed samples were performed with a JEOL screening to identify the amino acid. JLC-6AH amino acid analyser. The column for acidic and neutral amino acids contained JEOL Materials and Methods LCR2 resin. For radioactive samples, a stream splitter was used before the ninhydrin-analysing Pyruvate kinase modification system and approx. 10% of the eluate from the The exact conditions for the bulk modification of column was collected in a fraction collector. The pyruvate kinase were described in the previous paper entire sample was counted for radioactivity. Vol. 159 214 R. A. CHALKLEY AND D. P. BLOXHAM Performic acid oxidation vacuo. After acid hydrolysis, the amino acid analyser Performic acid was prepared by the addition of showed the presence of glutamate, glycine and a new H202 (30%, v/v) to formic acid (firs, 1956). In a amino acid, which was presumably compound I typical experiment, 25mg of 5-chloro-4-oxo[3,5-3H]- (Fig. 1). To confirm this, we purified the amino acid pentanoate-labelled material was dissolved in 0.5ml and identified its structure by n.m.r. spectroscopy. of formic acid, cooled on ice for 30min, andthen 1 ml Fortunately, the mixture of amino acids was easily of performic acid solution was added. After 2.5h at purified by high-voltage electrophoresis at pH3.5. 0WC, the sample was diluted with 25ml of water and After performing this chromatographic purification freeze-dried. The sample was hydrolysed and ana- twice, the modified amino acid gave only a single lysed by electrophoresis. N.-3-Carboxymethylhisti- ninhydrin-positive and radioactive peak in the dine and e-carboxymethyl-lysine (Gundlach et al., amino acid analyser. The n.m.r. spectrum of this 1959) were synthesized by Dr. G. Rasool of this compound was recorded in 2H20 by using a Varian department. XL100-12 machine operated in the CW mode at 100MHz with a "IF lock and extemal trimethylsilyl reference, The hydrogen resonances observed were as Periodate oxidation follows; 3 1.8-2.8p.p.m., 4H, multiplet, -CH2 The NaBH4-reduced 5-chloro4-oxo[3,5-3H]pen- CH2-; a 2.85p.p.m., doublet, S-CH2C; tanoate-labelled samples were oxidized with 100mg 2H, of NaIO4 in lOml of satd. NaHCO3 containing 02H 10mg ofcarrier formaldehyde. After 16h the niixture 3 3.05p.p.m., 2H, triplet, C-CH2-S; a 4.6-4.9 was adjusted to pH6 with 50% (w/v) acetic acid, and I formaldehyde was isolated as the dimedone complex N2H2 (Zaman et al., 1973). p.p.m., 1H, multiplet, --H. This spectrum ac- 5.Amino.4hydroxy[5-3H2]pentanoic acid was I donated by Dr. P. M. Jordan of this department. N2H2 counted for all the expected resonances. The methyl- Results and Discussion ene group adjacent to the amino group Synthesis and characterization of4-hydroxy[3,5_3H]- H-CH2-S in compound I is not a simple pentanoic acidalanine thioether (I) The first task in identifying the amino acid in \N2H2 pyruvate kinase modified by 5-chloro-4-oxopentano- ate was to synthesize an authentic reference amino acid. For this purpose we chose 4-hydroxypentanoic 1.0o In acid alanine thioether (I), since thealkylation reaction 0 was easily monitored by 5,5'-dithiobis-(2-nitro- V 0.8 8 benzoate) titration (Ellman, 1959). Further, gluta- 0in v: thione presents a useful N-blocked cysteine deriva- o 0.6 6 *5 tive, since its adducts are readily precipitated as the ._ barium salt, which aids purification. U U .* 0.4 4 .... OH NH2 .0 . 04 I ..r- 0.2 2 ,,x CO2H-[CH2]2-CH-CH2-S-CH2--CH-CO2H z o" (I) 120 140 160 180 200 220 240 260 Glutathione (0.3mmol) was allowed to react with Retention time (min) 5-chloro4oxo[3,5-3Hlpentanoate (0.42mmol; 2.5 x Fig. 1. Chromatography of 4-hydroxy(3,5-3-H]pentanoic 103d.p,m./nmol) in water, pH8 (maintained with acidalanine by using the amino acidanalyser IM-NaOH) for 2h at room temperature (20°3). About 80 % ofthereactionwascompletewithin 0min This Figure shows the chromatography of 4-hydroxy- as judged by both 5,5'-dithiobis-(2-nitrobenzoate) [3,5-3H]pentanoic acid alanine thioether, which was titration and alkali uptake. The carbonyl group in the produced either synthetically from glutathione or by alkylation of pyruvate kinase. The trace shows the elution adduct was then reduced with NaBH4 and after profile from only the acidic and neutral column of the 30min an excess of barium acetate (4m1, 1M) was amino acid analyser. o, Ninhydrin-positive trace whlen added. The barium salt was precipitated with 4vol. 5- chloro-4-oxo[3,5-3H]pentanoate-labelled glutathione of ice-cold ethanol and collected by centrifugation, was analysed. *, Distribution of radioactivity when After washing three times with ice-cold ethanol and 5-cWoro-4'oxo[3,5_3H]pentanoate-labelled pyruvate kin- twice with diethyl ether the product was dried in ase was analysed. 1976 ALKYLATION OF CYSTEINE BY 5-CHLORO-4-OXOPENTANOIC ACID 215 doublet, but is more complex. This may be due to the derived from position 3 of5-chloro-4-oxopentanoate) fact that it is adjacent to an asymmetric centre which and [3H]formaldehyde. This route gives no informa- was derived from the L-cysteine in glutathione. This tion on the nature of the amino acid modified. In may be contrasted with the methylene group adjacent contrast, route (b) leads to the formation of the [3H]- to the hydroxyl group (S-CH2-CH- which is a carboxymethyl-amino acid (eH derived from position 5 of 5-chloro-4-oxopentanoate), which can be k 02H1 identified by comparison with standards. [3H]- simple doublet, reflecting that the NaBH4 reduction Hydroxypropionate formed by route (b) will be of the carbonyl produces a racemic mixture. volatile and not analysed.