Involved in reduction steps of the biosynthesis of plant campesterol-derivative steroids, ending to castasterone (CS) but missing brassinolide (BL) (PubMed:32146811). Catalyzes the conversion of campest-4-en-3-one (campesta-4-en-3-one, 4-en-3-one) to campest-3-one (campesta-3-one, 3-one) and of (22S,24R)-22-hydroxyergost-4-en-3-one (22-hydroxy-campesta-4-en-3-one, 22-OH-4-en-3-one) to (22S,24R)-22-hydroxy-5alpha-ergostan-3-one (22-hydroxy-campesta-3-one, 22-OH-3-one) (PubMed:32146811). a 3-oxo-5alpha-steroid + NADP(+) = a 3-oxo-Delta(4)-steroid + H(+) + NADPH 5alpha-campestan-3-one + NADP(+) = campest-4-en-3-one + H(+) + NADPH (22S,24R)-22-hydroxy-5alpha-ergostan-3-one + NADP(+) = (22S)-22-hydroxycampest-4-en-3-one + H(+) + NADPH Plant hormone biosynthesis; brassinosteroid biosynthesis. Belongs to the steroid 5-alpha reductase family.