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

We have studied the hepatic microsomal 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) capacity of koala (Phascolarctos cinereus) and tammar wallab y (Macropus eugenii). A detailed comparison of the activity in hepatic frac tions from koala and rat was made. Hepatic microsomal NADP-supported 17 bet a-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 n moles/mg protein/min). However, when NAD was utilised as cofactor the activ ity 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 +/- 6.4 0 nmoles/mg protein/min, NAD; 2.18 +/- 1.04 nmoles/mg protein/min, NADP). M ichaelis-Menten parameters for the kinetics of 17 beta-HSD testosterone oxi dation by NADP and NAD were determined in the koala. The Km for testosteron e was of the order of 10.0-24.0 mu M (n = 6) irrespective of the cofactor u sed, whilst the Km for NADP was 0.28-0.43 mu M (n = 2) and for NAD was 13.9 -18.5 mu M (n = 2). 17 beta-estradiol was found to be an inhibitor of both NAD- and NADP- supported 17 beta-HSD activity. These findings indicate that NADP-mediated, but not NAD-mediated testosterone dehydrogenation is a majo r pathway of steroid biotransformation in koala liver: the reaction is less extensive in fractions from wallaby, human and rat. Such species-related d ifferences in cofactor preference may contribute along with species differe nces in gene expression to observed rates of 17 beta-HSD activity in mammal s. (C) 2000 Elsevier Science Inc. All rights reserved.