Cytochrome P450 isozymes catalyzing 4-hydroxylation of parkinsonism-related compound 1,2,3,4-tetrahydroisoquinoline in rat liver microsomes

Microsomal 4-hydroxylase of 1,2,3,4-tetrahydroisoquinoline (TIQ), a possible candidate for causing Parkinson disease, was characterized by using rat hepatic microsomes and purified P450 isozymes. Kinetic analysis revealed that K(m) and V(max) values (mean ± SE) for hepatic microsomal TIQ 4-hydroxylase of male Wistar rats were 319.6 ± 26.8 μM and 12.13 ± 1.43 pmol·min^-1·mg^-1 protein, respectively. When TIQ 4-hydroxylase activity was compared in Wistar (an animal model of extensive debrisoquine metabolizers) and Dark Agouti (an animal model of poor debrisoquine metabolizers) rats, significant strain (Wistar>Dark Agouti) and sex (male>female) differences were observed. The microsomal activity toward TIQ 4-hydroxylation was increased by pretreatment of male Wistar rats with P448 inducers (β-naphthoflavone and sudan I), but not with phenobarbital. Pretreatment with propranolol, an inhibitor of P450 isozymes belonging to the P450 IID gene subfamily, decreased TIQ 4-hydroxylase activity. P450 BTL, a P450 isozyme belonging to the IID subfamily, showed TIQ 4-hydroxylase activity of 64.1 pmol·min^-1·nmol P450^-1, which was 3.2-fold that of microsomes (20.9 pmol·min^-1·nmol P450^-1). Antibody (IgG) against this isozyme suppressed microsomal TIQ 4-hydroxylase activity concentration-dependently. A male-specific P450 ml (P450IIC11) catalyzed this reaction to a much lesser extent (10.0 pmol·min^-1·nmol P450^-1), and its antibody did not affect the microsomal activity. These results suggest that TIQ 4-hydroxylation in hepatic microsomes are catalyzed predominantly by a P450 isozyme (or isozymes) belonging to the IID gene subfamily in non-treated rats and its immunochemically related P450 isozyme (or isozymes), and that a P450 isozyme (or isozymes) belonging to the IA subfamily also participates in TIQ 4-hydroxylation in rats pretreated with P448-inducers.