The 1 '-hydroxylation of rac-bufuralol by rat brain microsomes

The 1'-hydroxylation of rac-bufuralol, which is catalyzed by polymorphic CY P2D6 in humans, was studied in brain microsomes from male and female Wistar rats and from the female Dark Agouti rat, a model of the CYP2D6 poor metab olizer phenotype. The kinetics of the 1'-hydroxylation of bufuralol (1-1500 mu M) by brain microsomes were biphasic. The activity of the high-affinity site of metabolism was consistent with Michaelis-Menten kinetics (apparent K-m1 = 0.61-1.42 mu M, V-max1 = 4.3-4.8 fmol/min/mg of protein), whereas t he low-affinity activity was better described by a Hill function (K-50%(2) = 253-258 mu M, V-max2 = 817-843 fmol/min/mg of protein, n = 1.2-1.3). Valu es for kinetic constants were similar in all rat strains. Quinine was only a weak inhibitor of both the high- (apparent K-i = 90 mu M) and low-affinit y (210 mu M) sites of metabolism. In contrast, the kinetics of 1'-hydroxyla tion of bufuralol by rat liver microsomes were best described by a two-site Michaelis-Menten function. V-max values were 3 to 5 orders of magnitude gr eater compared with those for brain microsomes (male and female Wistar), an d liver microsomes from female Dark Agouti rats were significantly less act ive than those from Wistar rats. These data, together with the known potent inhibitory effect of quinine on bufuralol 1'-hydroxylation by rat liver mi crosomes, indicate tissue-specific differences in the enzymology of this re action. The role of brain CYP2D enzymes remains to be clarified.