ACTIVATION OF DELTA-5-3-KETOSTEROID ISOMERASE OF BOVINE ADRENAL MICROSOMES BY SERUM ALBUMINS

The .DELTA.5-3-ketosteroid isomerase (EC 5.3.3.1) of bovine adrenal microsomes is activated as much as 10- to 20-fold by micromolar concentrations of bovine serum albumin [BSA]. Comparable activations are observed with the serum albumins of 10 other mammalian species, but are not seen with ovalbumin or conalbumin. Evidence that the activation is attributable to the serum albumins, rather than to a small, firmly-bound ligand, is based on: extraction, dialysis, or gel filtration; destruction of the activity by heating or by trypsin digestion; precipitation of the stimulatory activity of bovine serum albumin by specific antibody. BSA induces small decreases in the Km for .delta.5-androstene-3,17-dione, but most of the activational effect reflects an increase in the maximum velocity. Low concentrations of Triton X-100, which are without eftect on the isomerase activity, prevent the activation by BSA.