Suicide inactivation of cytochrome p450 by midchain and terminal acetylenes: A mechanistic study of inactivation of a plant lauric acid ω- hydroxlyase

Incubation of Vicia sativa microsomes, containing cytochrome P450- dependent lauric acid ω-hydroxylase (ω-LAH), with [1-¹⁴C]11-dodecynoic acid (11-DDYA) generates a major metabolite characterized as 1,12- dodecandioic acid. In addition to time- and concentration-dependent inactivation of lauric acid and 11-DDYA oxidation, irreversible binding of 11-DDYA (200 pmol of 11-DDYA bound/mg of microsomal protein) at a saturating concentration of 11-DDYA was observed. SDS-polyacrylamide gel electrophoresis analysis showed that 30% of the label was associated with several protein bands of about 53 kDa. The presence of β-mercaptoethanol in the incubate reduces 1,12-dodecandioic acid formation and leads to a polar metabolite resulting from the interaction of oxidized 11-DDYA with the nucleophile. Although the alkylation of proteins was reduced, the lauric acid ω- hydroxylase activity was not restored, suggesting an active site-directed inactivation mechanism. Similar results were obtained when reconstituted mixtures of cytochrome P450 from family CYP4A from rabbit liver were incubated with 11-DDYA. In contrast, both 11- and 10-DDYA resulted in covalent labeling of the cytochrome P450 2B4 protein and irreversible inhibition of activity. These results demonstrate that acetylenic analogues of substrate are efficient mechanism-based inhibitors and that a correlation between the position of the acetylenic bond in the inhibitor and the regiochemistry of cytochromes P450 oxygenation is essential for enzyme inactivation.