Cotinine in Human Liver Microsomes1

Nicotine is primarily metabolized to cotinine, and cotinine is further metabolized to trans-3’-hydroxycotinine in human liver, which is a major metabolite of nicotine in humans. We studied the formation of trans-3’-hydroxycotinine from cotinine in hu-man liver microsomes. trans-3’-Hydroxycotinine formation demonstrated single enzyme Michaelis-Menten kinetics (Km, 234.5 ± 26.8 M; Vm, 37.2 ± 2.4 pmoVmin/mg protein). Significant correlation (r =.967, P <.001) between cotinine 3’-hydroxylase activities at low (50 M) and high (1 mM) coti-nine concentrations in 20 human liver microsomes suggested the contribution of a single enzyme to cotinine 3’-hydroxylation. The cotinine 3’-hydroxylase activity correlated significantly with immunoreactive cytochrome P450 (CYP)2A6 contents (r =.756, P <.01) and coumarin 7-hydroxylase activity (r =.887, P <.001). The cotinine 3’-hydroxylase activity was inhibited b