Involved in cardenolide biosynthesis. Catalyzes the stereospecific conversion of progesterone to 5-beta-pregnane-3,20-dione. Can use progesterone, testosterone, 4-androstene-3,17-dione, cortisol and cortisone as substrates, but not pregnenolone, 21-OH-pregnenolone or isoprogesterone. NADPH could not be replaced by NADH. 5beta-cholestan-3-one + NADP(+) = cholest-4-en-3-one + H(+) + NADPH 4,5beta-dihydrocortisone + NADP(+) = cortisone + H(+) + NADPH Optimum pH is 7.8. Optimum temperature is 40 degrees Celsius. Homodimer. Belongs to the short-chain dehydrogenases/reductases (SDR) family. Highly divergent.