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

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Abstract
The role of phospholipid in the beef adrenal microsomal 3‐oxosteroid δ4—δ5‐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 δ4—δ5‐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 δ4—δ5‐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 δ4—δ5‐isomerase activity in the lipid‐depleted membranes. It is concluded that the bovine adrenal microsomal 3‐oxosteroid δ4—δ5‐isomerase requires phospholipid(s) to exhibit its full catalytic activity.

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