Testosterone dehydrogenase activity in koala liver: characterisation of cofactor and steroid substrate differences

We have studied the hepatic microsomal 17beta-hydroxysteroid dehydrogenase (17beta-HSD) capacity of koala (Phascolarctos cinereus) and tammar wallaby (Macropus eugenii). A detailed comparison of the activity in hepatic fractions from koala and rat was made. Hepatic microsomal NADP-supported 17beta-HSD activity was significantly higher in koala (11.64+/-3.35 nmoles/mg protein/min), (mean+/-S.D.) than in tammar wallaby liver (1.52+/-0.79 nmoles/mg protein/min). However, when NAD was utilised as cofactor the activity was similar in both marsupial species (2.83+/-2.03 nmoles/mg protein/min, koala; 0.70+/-0.71 nmoles/mg protein/min, tammar wallaby). Data for rat indicated a cofactor preference for NAD rather than NADP (17.94+/-6.40 nmoles/mg protein/min, NAD; 2.18+/-1.04 nmoles/mg protein/min, NADP). Michaelis-Menten parameters for the kinetics of 17beta-HSD testosterone oxidation by NADP and NAD were determined in the koala. The Km for testosterone was of the order of 10.0-24.0 microM (n = 6) irrespective of the cofactor used, whilst the Km for NADP was 0.28-0.43 microM (n = 2) and for NAD was 13.9-18.5 microM (n = 2). 17beta-estradiol was found to be an inhibitor of both NAD- and NADP- supported 17beta-HSD activity. These findings indicate that NADP-mediated, but not NAD-mediated testosterone dehydrogenation is a major pathway of steroid biotransformation in koala liver; the reaction is less extensive in fractions from wallaby, human and rat. Such species-related differences in cofactor preference may contribute along with species differences in gene expression to observed rates of 17beta-HSD activity in mammals.