Mechanism of action of the porphyrogenic agent ATMP and the identification of griseofulvin induced green pigments in the mouse.

Hepatic protoporphyria is a disorder of liver haem metabolism characterised by inhibition of the enzyme ferrochelatase (FK) and consequent accumulation of protoporphyrin. Two compounds that induce such a disorder are ATMP [1-[4-(acetyl-2,4,6-trimethylphenyl)-2,6-cyclohexanedionyl]-O-ethyl propionaldehyde oxime] and griseofulvin. These drugs are thought to be metabolised by cytochrome P450 as suicide substrates so that a fragment of the drug (a methyl group) becomes attached to the haem moiety of the enzyme, producing a powerful inhibitor of FK. The present work was undertaken to elucidate the mechanism of action of ATMP and further characterise the N-alkylated porphyrins (green pigments) produced by feeding mice griseofulvin. ATMP causes hepatic protoporphyria in mice but not in similarly treated rats, guinea-pigs, hamsters and chick embryos. A green pigment was isolated from the liver of mice treated with ATMP and identified by its electronic absorption spectrum and HPLC chromatographic properties as N-methylprotoporphyrin (N-MePP). The ATMP pigment markedly inhibited the enzyme FK in vitro, thus supporting its identification as N-MePP. The possible involvement of cytochrome P450 in the production of N-MePP after treatment with ATMP was also investigated. Loss in cytochrome P450 was not demonstrable in mice treated with ATMP. However, a role for cytochrome P450 was suggested by the findings that two inhibitors of cytochrome P450, SKF525-A and piperonyl butoxide, both afforded protection against ATMP-induced porphyria and production of N-MePP. In the mouse griseofulvin causes suicidal destruction of cytochrome P450 giving rise to green pigments in the liver. Two such pigments have been characterised here, using fast atom bombardment (FAB) mass spectrometry. One of them has been conclusively identified as N-MePP; the second is also an N-alkylated protoporphyrin and has been tentatively identified as the adduct of griseofulvin to protoporphyrin (N-griseofulvin protoporphyrin).