COMPARISON OF CYP2A6 CATALYTIC ACTIVITY ON COUMARIN 7-HYDROXYLATION IN HUMAN AND MONKEY LIVER-MICROSOMES

Comparison of 7-hydroxylation of coumarin, a CYP2A6 substrate, in huma n and African green and cynomolgus monkey liver microsomes was made by means of an HPLC assay with UV detection. In human liver microsomes, the K-m and V-max values for the metabolic conversion were 2.1 mu M an d 0.79 nmol/mg/min, respectively. While African green monkey showed K- m and V-max values of 2.7 mu M and 0.52 nmol/mg/min, which were simila r to human, higher K-m and V-max values were found in cynomolgus monke y. Coumarin 7-hydroxylation in human and African green monkey was sele ctively inhibited by methoxsalen and pilocarpine (CYP2A6 inhibitors) b ut not by other inhibitors, i.e. alpha-naphthoflavone (CYP1A1), orphen adrine (CYP2B6), sulfaphenazole (CYP2C9), quinidine (CYP2D6) and ketoc onazole (CYP3A4). Immunoinhibition results supported CYP2A6 involvemen t in human and its homolog in monkey in coumarin 7-hydroxylation, as o nly anti-CYP2A6, but not CYP2B1, CYP2C13, CYP2D6, CYP2E1 or CYP3A anti bodies, inhibited this conversion. African green monkey was found to b e similar to human in catalytic activity of coumarin 7-hydroxylation a nd response to CYP2A6 inhibitors or antibody inhibition: However, the monkey CYP2A6 is not identical to the human in that K-i values were di fferent, and differences were observed with some CYP2A6 inhibitors, su ch as nicotine and methoxsalen, suggesting that, under some circumstan ces, studies of nicotine kinetics and drug taking behavior in monkey m ay not be comparable to human.