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

		+		NADP+	=			+		NADPH
	3alpha,17alpha,21-trihydroxy-5beta-pregnan-11,20-dione	+		NADP+	=		17alpha,21-dihydroxy-5beta-pregnan-3,11,20-trione	+		NADPH