Isolation of an N-alkylprotoporphyrin IX from chick embryo livers following the administration of 3,5-diethoxycarbonyl-1,4-dihydro-4-ethyl-2,6-dimethylpyridine

Isolation of regioisomers of N-alkylprotoporphyrin IX from chick embryo liver after treatment with porphyrinogenic xenobiotics

Several porphyrinogenic xenobiotics cause mechanism-based inactivation of cytochrome P450 (P450) isozymes with concomitant formation of a mixture of four N-alkylprotoporphyrin IX (N-alkylPP) regioisomers, which have ferrochelatase inhibitory properties. To isolate the four regioisomers of N-methylprotoporphyrin IX (N-methylPP), 3,5-diethoxycarbonyl, 1-4-dihydro-2,4,6-trimethylpyridine (DDC) was administered to untreated, β-naphthoflavone-, phenobarbital-, and glutethimide-pretreated 18-day-old chick embryos. Separation of the N-methylPP regioisomers by high pressure liquid chromatography (HPLC) revealed no marked difference in the regioisomer pattern among the different treatments. After administration of griseofulvin, allylisopropylacetamide (AIA), or 1-[4-(3-acetyl-2,4,6-triemethylphenyl)-2,6-cyclohexanedionyl]-O-ethyl propionaldehyde oxime (ATMP) to untreated and glutethimide-pretreated 18-day-old chick embryos, an N-alkylPP was isolated after AIA administration only. This finding strengthened previous reports of the species specificity of N-alkylPP formation with griseofulvin and ATMP. A series of dihydropyridines, namely 4-ethylDDC, 4-hexylDDC, and 4-isobutylDDC were administered to untreated and glutethimide-pretreated 18-day-old chick embryos and hepatic N-alkylPPs were isolated and separated by HPLC into regioisomers. The regioisomer patterns obtained did not support a previous proposal of masked regions above both rings B and C in the heme moieties of the P450 isozymes responsible for N-alkylPP formation. However, the data support the hypothesis of a partially masked region above ring B alone. The regioisomer patterns were in agreement with results previously obtained in rats showing that the percentage of NCand (or) NDregioisomers in the regioisomer mixture increases as the length and bulk of the 4-alkyl substituent of a DDC analogue increase. Differences in the regioselectivity of heme N-alkylation may be due to intrinsic chemical features of DDC analogues themselves or to differences in the P450 isozymes inactivated.Key words: mechanism-based inactivation, cytochrome P450, N-alkylprotoporphyrin IX, experimental porphyria, dihydropyridine.

Evidence for mechanism-based inactivation of rat and chick embryo hepatic cytochrome P4501A and P4503A by dihydropyridines, sydnones, and dihydroquinolines

Rat hepatic P4501A1 and 3A1/2 have been shown previously to be targets for mechanism-based inactivation by the 4-alkyl analogues of 3,5-diethoxycarbonyl-1,4-dihydro-2,4,6-trimethylpyridine (DDC), namely, 4-ethyl DDC and 4-isopropyl DDC. In this study we have shown that rat hepatic P4501A and P4503A are targets for mechanism-based inactivation by the sydnones, 3-[2-(2,4,6-trimethylphenyl)thioethyl]-4-methylsydnone (TTMS) and 3-(2-phenylethyl)-4-methylsydnone (PEMS). The dihydroquinoline, 2,4-diethyl-2-methyl-1,2-dihydroquinoline (DMDQ), caused mechanism-based inactivation of rat hepatic P4501A but not of P4503A. The P4501A isozyme(s) of chick embryo liver was found to share the ability of rat liver P4501A to serve as a target for mechanism-based inactivation by the dihydropyridines, 4-ethyl DDC and 4-isopropyl DDC, the sydnones, TTMS and PEMS, and the dihydroquinoline, DMDQ. A P4503A-like isozyme of chick embryo liver shared the ability of the rat liver P4503A isozyme(s) to serve as a target for mechanism-based inactivation by the dihydropyridines, 4-ethyl DDC and 4-isopropyl DDC, and the sydnone, TTMS, but not of the sydnone PEMS. The dihydropyridine, DDC, was found to serve as a mechanism-based inactivator of the chick embryo P4501A isozyme(s), but not of the P4503A isozyme(s), in contrast to its previously reported inactivity with both the rat hepatic P4501A1 and 3A1/2 isozymes.

Effects of 3-(2-phenylethyl)-4-methylsydnone and related sydnones on heme biosynthesis

3-[2-(2,4,6-Trimethylphenyl)thioethyl]-4-methylsydnone (TTMS) and 3-(2-phenylethyl)-4-methylsydnone (PEMS) cause mechanism-based inactivation of rat hepatic microsomal cytochrome P-450 and the formation of N-alkylprotoporphyrins in rat liver. In the present study, we have shown that both TTMS and PEMS cause mechanism-based inactivation of chick embryo hepatic microsomal cytochrome P-450. TTMS also caused the inhibition of ferrochelatase activity, the accumulation of protoporphyrin IX, and an increase in the activity of delta-aminolevulinic acid synthase in chick embryo liver cell culture. PEMS was devoid of effect on ferrochelatase activity, porphyrin accumulation, and delta-aminolevulinic acid synthase activity. There are two possible explanations for the lack of effect of PEMS on heme biosynthesis: (1) the ring-A- and/or ring-B-substituted regiosomers of the N-phenylethyl- and N-phenylethenylprotoporphyrins which are produced during the mechanism-based inactivation of cytochrome P-450 by PEMS are too bulky to fit into the active site of ferrochelatase to inhibit its activity, in contrast to the N-vinylprotoporphyrin formed from TTMS; and (2) the N-alkylprotoporphyrins produced consist of the ring-C- and/or ring-D-substituted regioisomers, which are not inhibitors of ferrochelatase activity.

Patterns of porphyrin accumulation in response to xenobiotics. Parallels between results in chick embryo and rodents

Our objective was to determine whether patterns of porphyrin accumulation produced by chemicals in chick embryo hepatocyte culture would indicate which enzyme of heme biosynthesis was inhibited. The ferrochelatase-inhibitory potency and porphyrin patterns produced by DDC, TTMS, and their analogues were studied. The protoporphyrin:coproporphyrin ratio observed was found to correlate with ferrochelatase-inhibitory activity. The results obtained in chick embryo with TTMS and DDC parallel those found in rodents. Griseofulvin has been shown to lower ferrochelatase activity and to cause the accumulation of protoporphyrin in rodent liver. In chick embryo liver cell culture, however, coproporphyrin, uroporphyrin, and heptacarboxylic acid porphyrin accumulate and ferrochelatase activity is not lowered. Uroporphyrin, heptacarboxylic acid porphyrin, and coproporphyrin are the major porphyrins to accumulate in response to PAHs (for example, 3,3',4,4'-TCBP in chick embryo liver cell culture). This may be explained by inhibition of UROD, which has been observed in chick embryo and rodent liver. Some chemicals, such as phenobarbital and nifedipine, cause the accumulation of these porphyrins in chick embryo liver cell culture, and this is explained by inhibition of UROD. These chemicals have not been reported to interfere with heme biosynthesis in the intact chick embryo or rodents; possibly protective mechanisms that are not available in the cell culture system are operative in the intact animal. It was concluded that porphyrin patterns may serve as a guide to which enzyme of heme biosynthesis is inhibited in chick embryo liver cell culture. The results obtained in the culture system with certain chemicals, such as DDC and TTMS analogues and PAHs, correspond with results in rodents. In other cases, such as with griseofulvin, the results do not correspond.