CYTOCHROME-P450 MEDIATED BIOACTIVATION OF METHYLGLYOXAL TO 1'-HYDROXYMETHYLEUGENOL IN FISCHER-344 RAT AND HUMAN LIVER-MICROSOMES

Cytochrome P450 mediated metabolism of methyleugenol to the proximate carcinogen 1'-hydroxymethyleugenol has been investigated in vitro. Kin etic studies undertaken in liver microsomes from control male Fischer 344 rats revealed that this reaction is catalyzed by high affinity (K- m of 74.9 +/- 9.0 mu M, V-max of 1.42 +/- 0.17 nmol/min/nmol P450) and low affinity (apparent K-m several mM) enzymic components, Studies un dertaken at low substrate concentration (20 mu M) with microsomes from livers of rats treated with the enzyme inducers phenobarbital, dexame thasone, isosafrole and isoniazid indicated that a number of cytochrom e P450 isozymes can catalyze the high affinity component, In control r at liver microsomes, 1'-hydroxylation of methyleugenol (assayed at 20 mu M substrate) was inhibited significantly (P < 0.05) by diallylsulfi de (40%), p-nitrophenol (55%), tolbutamide (30%) and alpha-naphthoflav one (25%) but not by troleandomycin, furafylline, quinine or cimetidin e. These results suggested that the reaction is catalyzed by CYP 2E1 a nd by another as yet unidentified isozyme(s) (most probably CYP 2C6), but not by CYP 3A, CYP 1A2, CYP 2D1 or CYP 2C11. Administration of met hyleugenol (0-300 mg/kg/day for 5 days) to rats in vivo caused dose-de pendent auto-induction of 1'-hydroxylation of methyleugenol in vitro w hich could be attributed to induction of various cytochrome P450 isozy mes, including CYP 2B and CYP 1A2. Consequently, high dose rodent carc inogenicity studies are likely to overestimate the risk to human healt h posed by methyleugenol. The rate of 1'-hydroxylation of methyleugeno l irt vitro in 13 human liver samples varied markedly (by 37-fold), wi th the highest activities being similar to the activity evident in con trol rat liver microsomes. This suggests that the risk posed by dietar y ingestion of methyleugenol could vary markedly in the human populati on.