Kinetic properties of microsomal 3ß hydroxysteroid dehydrogenase 5-ene isomerase from the testis of Bufo arenarum H

POZZI AG; CANOSA LF; LANTOS CP; CEBALLOS N

JOURNAL OF STEROID BIOCHEMISTRY AND MOLECULAR BIOLOGY

Pergamon

Lugar: Nueva York; Año: 2000 vol. 73 p. 257 - 264

0960-0760

3b-hydroxysteroid dehydrogenase 5-ene isomerase (3bHSD: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 3bHSD:I has more affinity for dehydroepiandrosterone (DHEA) than for pregnenolone (Km_0.1790.03 and 1.02 mM, 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 Vmax while Km was not modified, suggesting a non-competitive inhibition of the conversion of pregnenolone. Ki was calculated from replot of Dixon’s slope for each substrate concentration. Ki from the intercept and the slope of this replot were similar (0.27690.01 and 0.26390.02 mM) and higher than the Km for DHEA. The Km and Ki 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 Vmax while Km 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.