Evidence for involvement of human CYP3A in the 3-hydroxylation of quinine

Aims Our previous studies using in vitro hepatic microsomal preparations 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 microsomal techniques were employed.
Results The mean apparent K _m value for 3-hydroxyquinine formation was 83±19 (s.d.)  μm, ranging from 57  μm to 123  μm 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 inhibitors, troleandomycin, midazolam and erythromycin. Inhibitors selective for CYP1A1/2, CYP2D6, CYP2E1, CYP2C9/10 or CYP2C19 had little or no effect on quinine 3-hydroxylation. Using microsomes from a panel of livers, significant correlations were found only between 3-hydroxyquinine activity and other CYP3A activities (caffeine 8-oxidation, omeprazole sulphoxidation, midazolam 1'-hydroxylation and midazolam 4-hydroxylation) and immunoreactive CYP3A content. There were no statistically significant correlations with activities selective for CYP1A2, CYP2C9 and CYP2E1. Competitive inhibition of quinine 3-hydroxylation was observed with a substrate known to be specifically metabolized by human CYP3A, i.e. midazolam, with an apparent K _i value of 11.0  μm.
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.