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N-Methylprotoporphyrin IX: Chemical Synthesis and Identification

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N-Methylprotoporphyrin IX: Chemical Synthesis and Identification Proc. Nati. Acad. Sci. USA Vol. 78, No. 3, pp. 1490-1494, March 1981 Biochemistry N-Methylprotoporphyrin IX: Chemical synthesis and identification as the green pigment produced by 3,5-diethoxycarbonyl-1,4- dihydrocollidine treatment (N-alkyl porphyrins/porphyrin synthesis/ferrochelatase inhibitor/NMR) PAUL R. ORTIZ DE MONTELLANO, HAL S. BEILAN, AND KENT L. KUNZE Department of Pharmaceutical Chemistry, School of Pharmacy, and Liver Center, University of California, San Francisco, California 94143 Communicated by Rudi Schmid, November 24, 1980 ABSTRACT The hepatic pigment accumulated in 3,5-di- mitochondrial ferrochelatase (13, 14). The differential physio- ethoxycarbonyl-1,4-dihydrocollidine-treated rats, which has been logical effects of AIA and DDC are thus related, at least in part, reported to inhibit ferrochelatase, has been isolated and purified. to differences in the structures of the accompanying The pigment has been resolved into one major, one minor, and hepatic two trace components, all of which appear to be isomeric por- pigments. In concert with our efforts to define the structure of phyrins. The major fraction has been unambiguously identified the abnormal porphyrins due to AIA and other unsaturated byspectroscopic methods as the isomer of N-methyprotopor- agents (9, 10, 15, 16), we have undertaken a structural inves- phyrin IX (isolated as the dimethyl ester) in which vinyl-substituted tigation of the DDC-induced pigment. We report here un- pyrrole ring A is methylated. The minor product appears to be an ambiguous spectroscopic identification of the DDC pigment isomer of the same porphyrin with the N-methyl group on pro- as N-methylprotoporphyrin IX (dimethyl ester), confirmation pionic acid-substituted ring C, and the trace components have the same high-pressure liquid chromatography retention times as the of this structural assignment by chemical synthesis of the four other two possible isomers of the porphyrin. The four isomers of possible isomers of the structure, and use of the synthetic iso- N-methylprotoporphyrin LX have been chemically synthesized, mers to define the ratio of isomers in the biological product. independently characterized, and used to confirm the structures of the biological products. MATERIALS AND METHODS The heuristic perturbation of hepatic heme synthesis by chem- Isolation of Abnormal Porphyrins. Sprague-Dawley male ical agents has played a pivotal role in elucidation of the heme rats weighing approximately 250 g received aqueous sodium biosynthetic pathway and of the mechanism through which it phenobarbital (80 mg/kg of body weight daily) intraperitoneally is regulated. Two agents, allylisopropylacetamide (AIA) and for 4 days prior to injection by the same route of DDC (400 mg/ 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC), have achieved kg) in dimethyl sulfoxide (100 mg/ml). The rats were decapi- particular prominence in this regard because the biosynthetic tated 4 hr after DDC injection and their livers were perfused derangements that they engender mimic in many respects the in situ and subsequently homogenized in ice-cold 0.9% saline inherited metabolic defects characteristic of certain human por- (1 ml of saline per liver). The resulting homogenate was added phyrias (1). Administration of DDC to animals results in inhi- to 5% (vol/vol) H2SO4methanol (100 ml per liver) and the bition of the hepatic mitochondrial ferrochelatase responsible mixture was stored at 40C in the dark for 17-20 hr. Filtration, for the insertion of iron into the porphyrin ligand and conse- addition of 0.5 vol of water, and extraction twice with meth- quently leads to the accumulation of protoporphyrin IX (2-4). ylene chloride yielded an organic extract, which was washed Direct inhibition of ferrochelatase by DDC, however, appears four times with equal volumes of water before 0.5 ml of meth- not to be involved because the enzyme is not inhibited in vitro anol saturated with zinc acetate was added. After drying over by this agent (2, 3). The mechanism of the inhibitory interaction anhydrous sodium sulfate and removal of the solvent on a rotary has therefore remained obscure. evaporator, a crude residue was obtained which was purified More than a decade ago, two independent groups observed by thin-layer chromatography on Analtech (1000 ,um) silica gel a rapid decrease in hepatic cytochrome P-450 levels in mice G plates using 3:1 (vol/vol) chloroform/acetone. The red-flu- treated with either AIA or DDC (5, 6). Subsequent studies es- orescing fraction (RF 0.4-0.6) was extracted with methanol and tablished that AIA-mediated loss of cytochrome P-450 reflected was rechromatographed under the same conditions. The puri- conversion of the enzyme prosthetic heme into an abnormal fied sample was then subjected to high-pressure liquid chro- green pigment (7, 8). We have established that the prosthetic matography (HPLC) on a 4.6 X 250 mm Partisil 10-PAC (What- heme is alkylated by an activated form of AIA produced during man) column that was eluted with a 20 min 0-100% linear catalytic processing of this substrate, andwe have demonstrated gradient of methanol into 1:1 (vol/vol) hexane/tetrahydro- that the AIA green pigment is a covalent adduct of AIA with furan. The zinc-complexed pigment, which migrated as a single protoporphyrin IX (9-11). The interaction of DDC with cyto- peak, was demetalated by passage through 5% (vol/vol) H2SOd chrome P-450, in contrast to the interaction of AIA with this methanol (10). HPLC of the metal-free sample on a 9.4 x 250 enzyme, has not been intensively investigated. Tephly, Gibbs, mm Partisil 10-PAC semipreparative column eluted isocrati- and De Matteis, however, have recently reported that a green cally with 97:97:6 (vol/vol) hexane/tetrahydrofuran/methanol hepatic pigment is also formed in DDC-treated mice (12, 13) resolved the porphyrin isomers present. These were collected and have made the further important discovery that this pig- separately and the principal isomer, used subsequently for ment, unlike that obtained with AIA, is a potent inhibitor of NMR studies, was further purified by reconversion to the zinc The publication costs of this article were defrayed in part by page charge Abbreviations: AIA, allylisopropylacetamide (2-isopropyl-4-penten- payment. This article must therefore be hereby marked "advertise- amide); DDC, 3,5-diethoxycarbonyl-1,4-dihydrocollidine; HPLC, high- ment" in accordance with 18 U. S. C. §1734 solely to indicate this fact. pressure liquid chromatography. 1490 Downloaded by guest on September 26, 2021 Biochemistry: Ortiz de Montellano et aL Proc. Nat. Acad. Sci. USA 78 (1981) 1491 were obtained on a modified Kratos MS-9 instrument. Electron II impact mass spectra were obtained on a Kratos MS-25 instru- ment at 70 eV. NMR spectra were taken at the University of ', California (Davis) NMR facility on a 360 MHz Nicolet NT-360 I Fourier transform NMR spectrometer. Generally, a 700 pulse was employed with a repetition time of 1.26 sec and an acqui- II sition time of 1.16 sec. Solutions of the porphyrins in deuter- ated chloroform were used for NMR studies. The zinc com- III A plexes were washed with sodium chloride solution prior to ramtu IV preparation of the solutions for NMR to ensure that chloride was the zinc counterion. All peaks are assigned relative to the chloroform signal at 7.21 ppm. Circular dichroism spectra were Cu3 w obtained in methylene chloride on a Jovan Dichrograph II. ascc co0 0 RESULTS The hepatic pigment formed in rats treated with DDC (400 mg/ kg), after carboxyl group methylation, extraction, and com- plexation with divalent zinc, was purified by sequential thin- /,f'\, layer chromatography and HPLC. A single band or peak was observed for the zinc complex throughout this purification se- quence. Removal of the zinc ion from the purified zinc complex, i however, followed by HPLC, led to resolution of the free base 2 6 10 14 18 into four relevant peaks (Fig. 1). The second and third of these Retentioin time, min peaks (in order of elution from the column) represent most of the isolated material. Of these two peaks, the former is quan- FIG. 1. HPLC analysis of the p)urified DDC pigment after demet- titatively the most important, whereas the first and fourth peaks alation but before fractionation (----) and the mixture of the four syn- represent relatively minor fractions. The ratio of the first two thetic isomers of the dimethyl estter of N-methylprotoporphyrin IX peaks to the latter two, all four being due to isomeric porphyrins (-). The synthetic isomers are numbered, in order of elution, as I-IV. Peaks with retention times of 2-7 min are due to impurities. complex (saturated methanolic zinc acetate) followed by HPLC purification as described above for the zinc complexes. Synthesis of the N-Methylprotoporphyrin IX (Dimethyl Ester) Isomers. Methylfluorosulfonate (1.0 ml, 12 mmol) and 500 mg of the dimethyl ester of protoporphyrin IX (0.84 mmol) in dry methylene chloride (50 ml) were stirred in the dark at room temperature for 3 days. After washing with water, drying over anhydrous sodium sulfate, and solvent removal, the prod- uct was chromatographed on a 4 x 37 cm column of 10% (wt/ 01) wt) water-deactivated Merck silica gel 60, using 20:1 (vol/vol) w chloroform/methanol as the eluting solvent. Unreacted starting cucc porphyrin (75 mg), the dimethyl ester of N-methylprotopor- a0 phyrin IX (250 mg), and the dimethyl ester of N,N-dimethyl- -o protoporphyrin IX (50 mg) were eluted, in that order, from the column. The N-monomethylated fraction was then resolved into the four possible isomers of N-methylprotoporphyrin IX by HPLC on a 9.4 X 250 mm Partisil 10-PAC column, using 97:97:6 (vol/vol) hexane/tetrahvdrofuran/methanol as eluting solvent.
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