The Enzymatic Reduction of A’-3-ketosteroids

Many previous studies (1) have indicated that cr,/?-unsaturated 3-ketosteroids are metabolized primarily by reductive reactions involving ring A. First, the double bond and then the carbonyl group are reduced, resulting in so called “tetrahydro” steroids. An earlier report from this laboratory (2) described an extract of rat liver which catalyzed both of these processes. The partial purification of the enzyme responsible for the second of these, i.e. the reversible reduction of the 3-keto group, was published previously (3). The present communication is concerned with the initial step in the sequence in which the 4,5 double bond of ring A is irreversibly hydrogenated by reduced pyridine nucleotide.