Formation of N-methyl protoporphyrin in chemically-induced protoporphyria. Studies with a novel porphyrogenic agent.

1-[4-(3-Acetyl-2,4,6-trimethylphenyl)-2,6-cyclohexanedionyl]-O-eth yl propionaldehyde oxime (for short ATMP) is a novel porphyrogenic agent causing hepatic protoporphyria in the mouse. Mice given a single dose of the drug showed 24 h later a 70% inhibition of liver ferrochelatase and marked accumulation of protoporphyrin. These changes were not seen in similarly treated rats, guinea pigs, hamsters or chick embryos. A green pigment was isolated from the liver of mice treated with ATMP and identified by its electronic absorption spectrum and chromatographic properties on HPLC as N-methyl protoporphyrin. The ATMP pigment markedly inhibited the enzyme ferrochelatase in vitro, thus supporting its identification as N-methyl protoporphyrin. Two inhibitors of liver cytochrome P450, compound SKF 525-A and piperonyl butoxide, when given before ATMP, afforded protection against ATMP-induced porphyria and production of N-methyl protoporphyrin, suggesting a role of cytochrome P450 in the induction of the metabolic disorder. The most likely interpretation for these findings is therefore that ATMP is metabolized in the mouse to a reactive species, which in turn alkylates the haem moiety of liver cytochrome P450, thus producing N-methyl protoporphyrin. This inhibits ferrochelatase and, as a secondary response, protoporphyrin accumulates. This pathway of metabolism to the postulated reactive metabolite presumably does not occur to a significant extent in the other species examined and hence is the likely basis for the species difference in protoporphyria.