Abstract

During the course of our research efforts to understand the kinetics of human aldehyde oxidase as a xenobiotic-clearing enzyme, we investigated the effect of eight different inhibitors on the oxidation of the probe substrate phthalazine. Saturation kinetic parameters for phthalazine oxidation in human liver cytosol were found to be the following: K_m = 8.0 ± 0.4 μM and V_max = 4.3 ± 0.1 nmol · min⁻¹ · mg protein⁻¹. Inhibitory potency of the inhibitors tested ranged from 0.1 to 5 μM. Of the eight different inhibitor compounds tested, seven were observed to inhibit through a mixed mode and one through a strictly competitive mode. A ratio of the K_ii and K_is values was used to assess the relative competitiveness of each inhibitor. For the mixed inhibitors, the mode of inhibition varied from mostly uncompetitive to predominantly competitive (K_ii/K_is values ranging from 0.1 to 15). The implications for potential drug-drug interactions and inhibition mechanism are discussed. We found two inhibitors, clozapine and chlorpromazine, that have a moderate predicted risk of drug-drug interactions based on the K_i value relative to the inhibitor concentration in human plasma, having a calculated [I]/K_i value of 0.4 and 0.8, respectively.