Reaction catalyzed by 3alpha-hydroxysteroid 3-dehydrogenase (Si-specific) (1.1.1.50)

		+		NADP+	=			+		NADPH
	3alpha,21-dihydroxy-5beta-pregnan-20-one	+		NADP+	=		21-hydroxy-5beta-pregnan-3,20-dione	+		NADPH