Lipid Requirement of Membrane‐Bound 3‐Oxosteroid δ⁴—δ⁵‐Isomerase

Abstract

The role of phospholipid in the beef adrenal microsomal 3-oxosteroid delta4-delta5-isomerase (EC 5.3.1.1) has been investigated with the use of phospholipase A to alter the microsomal phospholipids. The byproducts of phospholipase A digestion have been removed with a wash solution containing bovine serum albumin. Removal of 80-85% of the phospholipid leads to loss of 80-90% of the 3-oxosteroid delta4-delta5-isomerase activity. Reconstitution experiments have been performed by introduction of lipid aqueous dispersions in the enzymatic assay. Asolectin, a commercially available preparation of soy phosphatides, is able to stimulate the enzymatic activity but does not restore the 3-oxosteroid delta4-delta5-isomerase activity in phospholipase-A-treated membranes. In contrast, the introduction of aqueous dispersions of microsomal total lipid mixtures in the enzymatic assay brings about a complete restoration of the 3-oxosteroid delta4-delta5-isomerase activity in the lipid-depleted membranes. It is concluded that the bovine adrenal microsomal 3-oxosteroid delta4-delta5-isomerase requires phospholipid(s) to exhibit its full catalytic activity.