Metabolic activation and deactivation of dietary-derived coumarin mediated by cytochrome P450 enzymes in rat and human liver preparations

Dietary-derived coumarin is of clinical interest for its potential hepatotoxicity in humans because such toxicity is especially evident in rats. In this study, the oxidative metabolism of coumarin to active coumarin 3,4-epoxide (as judged by the formation rates of o-hydroxyphenylacetic acid) and excretable 7-hydroxycoumarin was investigated in liver fractions from rats and humans. In rat liver microsomes, the formation rate of o-hydroxyphenylacetic acid (~6 pmol/min/mg microsomal protein) from coumarin at 10 μM was dependent on the presence of liver cytosolic fractions. Rat hepatocytes mediated similar formation rates of o-hydroxyphenylacetic acid and 7-hydroxycoumarin (~0.1 nmol/hr/106 cells) at 0.20-20 μM coumarin. Human hepatocytes mediated the biotransformation of coumarin to o-hydroxyphenylacetic acid at roughly similar rates to those of rat hepatocytes. In contrast, the formation rates of 7-hydroxycoumarin by human hepatocytes were around 10-fold higher at ~1 nmol/hr/106 cells. In the presence of human liver cytosolic fractions, the oxidative formation rate of o-hydroxyphenylacetic acid was relatively high in cytochrome P450 (P450) 1A2-rich human liver microsomes. The inhibitory effects of furafylline/α-naphthoflavone and 8-methoxypsoralen, P450 1A2 and 2A6 inhibitors, respectively, were seen on the rates of o-hydroxyphenylacetic and 7-hydroxylation formations, respectively, in pooled human liver microsomes. Human liver microsomes selectively inactivated for P450 1A2 and 2A6 showed low rates of o-hydroxyphenylacetic acid and 7-hydroxylation formation (~20-30% of control), respectively. Among the P450 isoforms tested, recombinant human P450 1A2 predominantly mediated o-hydroxyphenylacetic formation. These results suggested that the metabolic activation and deactivation of coumarin were mediated mainly by P450 1A2 and 2A6 enzymes, respectively. The metabolic oxidation of coumarin via 3,4-epoxidation forming o-hydroxyphenylacetic acid could inform individual human risk assessments of dietary-derived coumarin, for which hepatotoxicity is especially evident in rats.