Kinetic properties of microsomal 3β hydroxysteroid dehydrogenase-isomerase from the testis of Bufo arenarum H

Abstract

3β-hydroxysteroid dehydrogenase 5-ene isomerase (3βHSD/I) activity is necessary for the biosynthesis of hormonally active steroids. A dual distribution of the enzyme was described in toad testes. The present study demonstrates that in testicular tissue of Bufo arenarum H., microsomal 3βHSD/I has more affinity for dehydroepiandrosterone (DHEA) than for pregnenolone ( K m=0.17±0.03 and 1.02 μM, respectively). The Hill coefficient for the conversion of DHEA and pregnenolone were 1.04 and 1.01, respectively. The inclusion of DHEA in the kinetic analysis of pregnenolone conversion affected V max while K m was not modified, suggesting a non-competitive inhibition of the conversion of pregnenolone. K i was calculated from replot of Dixon's slope for each substrate concentration. K i from the intercept and the slope of this replot were similar (0.276±0.01 and 0.263±0.02 μM) and higher than the K m for DHEA. The K m and K i values suggest the presence of two different binding sites. When pregnenolone was present in the assays with DHEA as substrate, no effect was observed on the V max while K m values slightly increased with pregnenolone concentration. Consequently, pregnenolone inhibited the transformation of DHEA in a competitive fashion. These studies suggest that, in this species, the microsomal biosyntheses of androgens and progesterone are catalysed by different active sites.