Differential effects of phenobarbitone and 3-methylcholanthrene induction on the hepatic microsomal metabolism of the β-carbolines harmine and harmol

Abstract

The metabolism of the β-carbolines harmine and harmol by C57/BL10 mouse liver microsomes is reported. Marked changes in apparent Michaelis-Menten kinetics and metabolite profiles were induced differentially by phenobarbitone (PB) or 3-methylcholanthrene (MC). With control or PB-induced microsomes harmine was metabolised by both high- and low-affinity reactions (app. K m = 1 and 29 μM for controls; app. K m = 1.9 and 21 μM for PB-induced). Only the high-affinity reaction occurred following MC pretreatment, induced 31-fold to V max = 22 nmoles·min −1·mg protein −1 (app. K m = 0.4 μM). With control or PB-induced microsomes harmine was metabolised almost exclusively by O-demethylation to harmol (3-fold induction to V max = 9.8 nmoles·min −1·mg protein −1, app. K m = 73 and 14 μM respectively), which was probably the low-affinity reaction of harmine metabolism. With MC-induced microsomes harmine was metabolised mainly to two unidentified yellow products, with harmol as a minor metabolite formed by a high-affinity reaction (app. K m = 0.6 μM). Control, PB- and MC-induced microsomes each metabolised harmol by a high-affinity reaction (app. K m = 0.6–1.9 μM) to unidentified metabolites. Harmol metabolism was not induced by PB, but was induced by MC (22-fold to Vmax = 11 nmoles·min −1·g protein −1): this partly explains the relative lack of net harmol production after MC induction. The unidentified MC-induced yellow metabolites of harmine were not formed via harmol. Harmine and harmol metabolism showed the characteristics of cytochrome P-450 catalysed reactions. Harmine gave Type II cytochrome P-450 binding spectra with control and PB-induced microsomes, but a high-affinity Type I spectrum ( K s = 6.5 μM) with MC-induced microsomes. Harmol gave modified Type II spectra in all cases, with again higher-affinity binding to MC-induced microsomes ( K s = 62 μM) than to control ( K s ⪢ 500 μM) or PB-induced microsomes ( K s = 500 μM).