Heterogeneity of membrane-bound delta5-3-oxosteroid isomerase. Studies on bovine adrenocortical microsomes.

There are conflicting reports concerning the number of distinct delta5-3-oxosteroid isomerases present in various steroid producing tissues; this problem was reexamined by two techniques: (1) kinetics measurements with a mixture of androst-5-ene-3,17-dione and pregn-5-en-3, 20-dione. (2) thermal inactivation of the androst-5-ene-3,17-dione isomerase and pregn-5-ene-3,20-dione isomerase activities. The results are the following: (a) Kinetic data support the concept of a single delta5-3-oxosteroid isomerase with a low substrate specificity in the microsomes. (b) Thermal inactivation experiments also support the one enzyme theory, but strongly suggest an heterogeneity of the delta5-3-oxosteroid isomerase in the bovine adrenocortical microsomes. In view of these results, a tentative explanation of the conflicting reports is proposed.