Catalysis of the reaction: a 3-oxo-5beta-steroid + NADP+ = a 3-oxo-Delta(4)-steroid + H+ + NADPH. The enzyme from human efficiently catalyzes the reduction of progesterone, androstenedione, 17alpha-hydroxyprogesterone and testosterone to 5beta-reduced metabolites; it can also act on aldosterone, corticosterone and cortisol, but to a lesser extent. The bile acid intermediates 7alpha,12alpha-dihydroxy-4-cholesten-3- one and 7alpha-hydroxy-4-cholesten-3-one can also act as substrates.
Delta(4)-5-beta-reductase activity