Plays a key role in early steps of protein N-linked glycosylation by being required for the conversion of polyprenol into dolichol. Dolichols are required for the synthesis of dolichol-linked monosaccharides and the oligosaccharide precursor used for N-glycosylation. Acts as a polyprenol reductase that promotes the reduction of the alpha-isoprene unit of polyprenols into dolichols in a NADP-dependent mechanism. Also able to convert testosterone (T) into 5-alpha-dihydrotestosterone (DHT). di-trans,poly-cis-dolichol + NADP(+) = di-trans,cis-polyprenol + H(+) + NADPH a 3-oxo-5alpha-steroid + NADP(+) = a 3-oxo-Delta(4)-steroid + H(+) + NADPH androst-4-ene-3,17-dione + H(+) + NADPH = 5alpha-androstan-3,17-dione + NADP(+) 17beta-hydroxy-5alpha-androstan-3-one + NADP(+) = H(+) + NADPH + testosterone Protein modification; protein glycosylation. Belongs to the steroid 5-alpha reductase family. Polyprenol reductase subfamily.