Destruction of cytochrome P-450 by 2-isopropyl-4-pentenamide and methyl 2-isopropyl-4-pentenoate: Mass spectrometric characterization of prosthetic heme adducts and nonparticipation of epoxide metabolites

Abstract

Incubation of hepatic microsomes from phenobarbital-treated rats with methyl 2-isopropyl-4-pentenoate results in rapid destruction of the microsomal cytochrome P-450. The destruction does not occur in the absence of NADPH or with methyl 2-isopropylpentanoate. Administration of methyl 2-isopropyl-4-pentenoate to phenobarbital-pretreated rats leads to hepatic accumulation of a “green” pigment which, after methylation and purification, yields an abnormal porphyrin chromatographically and spectroscopically indistinguishable from that similarly obtained with 2-isopropyl-4-pentenamide (allylisopropylacetamide). Field desorption mass spectrometry showed that both abnormal porphyrins exhibited molecular ions at m e 730. The mass spectrum of the zinc and copper complexes confirmed this value. Esterification in deuterated methanol of the amide-derived porphyrin showed that only two methyl esters were formed. Finally, methyl 4,5-epoxy-2-isopropylpentanoate and the known metabolites of 2-isopropyl-4-pentenamide were shown not to destroy cytochrome P-450. These results clearly establish that the carbonyl groups of the two destructive substrates are intimately involved in formation of the isolated porphyrin adducts, and exclude participation of the corresponding epoxide metabolites in the destruction of cytochrome P-450.