Species and Phenobarbitone-Induced Differences in the Kinetic Constants of Liver Microsomal Harmine O-Demethylation

Abstract

1. The apparent kinetic constants for the O-demethylation of harmine to harmol by 10 000 g supernatant fractions from livers of mice, rats, guinea-pigs, rabbits, cats and cows have been determined. The K_m values were 10–39 μM and V_max 0.25 and 1.65 nmol/mg protein/min.

2. Optimal conditions of incubation time and NADP requirements differed between species. In all species except cat and cow the rate of O-demethylation of harmine was linear for 5 min, but in the latter species was linear for 15 min. Maximum stimulation of O-demethylation occurred at NADP concn. of between 50 and 375 μm.

3. Phenobarbitone pre-treatment of weanling, young adult and mature adult mice increased the V_max for O-demethylation by 2.9- to 4.6-fold but did not change K_m. Increased V_max values were greatest in young and least in old mice and these changes were directly correlated with a decrease of hexobarbitone sleeping time.