title: Hepatic microsomal enzyme activity in the koala and tammar wallaby: high 17beta-hydroxysteroid oxidoreductase activity in koala liver microsomes.
abstract: We have studied the hepatic microsomal xenobiotic metabolising capacity of koala (Phascolarctos cinereus) and tammar wallaby (Macropus eugenii). Total cytochrome P450 content in hepatic microsomes from koala (0.87 +/- 0.18 nmol/mg protein, n = 4, mean (S.D.) and rat were comparable while tammar wallaby displayed reduced P450 content (0.24 +/- 0.04 nmol/mg protein). Associated microsomal activities (NADPH cytochrome P450 reductase, aminopyrine N-demethylation, aniline hydroxylation, and androstenedione 6beta- and 16alpha-hydroxylation) in koala liver were similar to or reduced relative to rat. Hepatic microsomal NADPH-supported 17beta-hydroxysteroid oxidoreductase (17beta-HSOR) activity was significantly higher in koala (9.99+/-3.08 nmol/mg protein/min) than in tammar wallaby liver (0.86 +/- 0.16 nmol/mg protein/min). However, when NADH was utilised as cofactor the activity was similar in both marsupial species (koala, 1.44 +/- 0.84 nmol/mg protein/min; tammar wallaby, 1.52 +/- 0.44 nmol/mg protein/min). Michaelis-Menten parameters for the kinetics of 17beta-HSOR androstenedione reduction by NADPH and NADH were determined in the koala. The Km for androstenedione was of the order of 1.9-4 microM (n = 4) irrespective of the cofactor used, whilst the Km for NADPH was 0.04-0.05 microM (n = 2) and for NADH was 134-430 microM (n = 2). Potential inhibitors were evaluated for their effects on NADPH-mediated 17beta-HSOR activity with menadione and, to lesser extents, menthone, benzaldehyde and metyrapone eliciting significant inhibition. From detailed kinetic studies menthone was found to be an uncompetitive inhibitor of the activity in koala liver (Ki 220 microM).
authors.affiliation: Center for Pharmaceutical Research, School of Pharmacy and Medical Sciences, University of South Australia, Adelaide, Australia. ieva.stupans@unisa.edu.au
authors.forename: I
authors.initials: I
authors.lastname: Stupans
authors.forename: S
authors.initials: S
authors.lastname: Kong
authors.forename: A
authors.initials: A
authors.lastname: Kirlich
authors.forename: M
authors.initials: M
authors.lastname: Murray
authors.forename: E L
authors.initials: EL
authors.lastname: Bailey
authors.forename: B R
authors.initials: BR
authors.lastname: Jones
authors.forename: R A
authors.initials: RA
authors.lastname: McKinnon
mesh_terms: D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D000072467:Cytochrome P450 Family 2; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D006913:Hydroxysteroid Dehydrogenases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D007614:Macropodidae / Q000378:metabolism*; D008297:Male; D008394:Marsupialia / Q000378:metabolism*; D008862:Microsomes, Liver / Q000201:enzymology*; D009249:NADP / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013045:Species Specificity; D039181:Steroid 16-alpha-Hydroxylase
chemical_list: D004791:Enzyme Inhibitors; D009249:NADP; D003577:Cytochrome P-450 Enzyme System; D006913:Hydroxysteroid Dehydrogenases; C035507:3(17)-hydroxysteroid dehydrogenase; D001189:Aryl Hydrocarbon Hydroxylases; C450279:CYP2C11 protein, rat; D000072467:Cytochrome P450 Family 2; D039181:Steroid 16-alpha-Hydroxylase