EVIDENCE FOR INVOLVEMENT OF HUMAN CYP3A IN THE 3-HYDROXYLATION OF QUININE

Aims Our previous studies using in vitro hepatic microsomal preparatio ns suggested that the hepatic metabolism of quinine to form the major metabolite 3-hydroxyquinine is most likely catalysed by human P450 3A (CYP3A). The present study was carried out to investigate the kinetics and to identify and further characterise the human liver CYP isoforms involved in the metabolism of quinine. Methods In vitro human microso mal techniques were employed. Results The mean apparent K-m value for 3-hydroxyquinine formation was 83 +/- 19 (s.d.) mu M, ranging from 57 mu M to 123 mu M in microsomes from ten human livers. There was a 6.7- fold variation in V-max values (mean 547 +/- 416 pmol min(-1) mg(-1)). Quinine 3-hydroxylation was inhibited by the specific CYP3A inhibitor s, troleandomycin, midazolam and erythromycin. Inhibitors selective fo r CYP1A1/2 CYP2D6, CYP2E1, CYP2C9/10 or CYP2C19 had little or no effec t on quinine 3-hydroxylation. Using microsomes from a panel of livers, significant correlations were found only between 3-hydroxyquinine act ivity and other CYP3A activities (caffeine 8-oxidation, omeprazole sul phoxidation, midazolam 1'-hydroxylation and midazolam 4-hydroxylation) and immunoreactive CYP3A content. There were no statistically signifi cant correlations with activities selective for CYP1A2, CYP2C9 and CYP 2E1. Competitive inhibition of quinine 3-hydroxylation was observed wi th a substrate known to be specifically metabolized by human CYP3A, i. e. midazolam, with an apparent K-i value of 11.0 mu M. Conclusions The present results strongly indicate that the conversion of quinine to 3 -hydroxyquinine is the major metabolic pathway in human liver in vitro and that the reaction is catalysed by CYP3A isoforms.