Involved in a reduction step in the biosynthesis of the plant steroid, brassinolide (BL) (PubMed:15993049). Can use progesterone, testosterone, androstenedione and campestenone as substrate (PubMed:15993049). a 3-oxo-5alpha-steroid + NADP(+) = a 3-oxo-Delta(4)-steroid + H(+) + NADPH Repressed by steroid (4-MA, VG106, PD91, PD17, Finasteride) and non-steroid (AS601811, AFA27, AFA76, AFA131, AFA192) inhibitors; steroid inhibitors are generally more efficient. Plant hormone biosynthesis; brassinosteroid biosynthesis. Mostly expressed in leaves and hypocotyls and, to a lower extent, in stems, cotyledons, roots, seeds and callus. Belongs to the steroid 5-alpha reductase family.