Plays a key role in early steps of protein N-linked glycosylation by being required for the conversion of polyprenol into dolichol (PubMed:20637498). Dolichols are required for the synthesis of dolichol-linked monosaccharides and the oligosaccharide precursor used for N-glycosylation (PubMed:20637498). Acts as a polyprenol reductase that promotes the reduction of the alpha-isoprene unit of polyprenols into dolichols in a NADP-dependent mechanism (PubMed:20637498). Also able to convert testosterone (T) into 5-alpha-dihydrotestosterone (DHT) (PubMed:17986282, PubMed:26855069). 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. Expressed in preadipocytes (at protein level) (PubMed:26855069). Overexpressed in hormone-refractory prostate cancers (HRPC). Almost no or little expression in normal adult organs. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the steroid 5-alpha reductase family. Polyprenol reductase subfamily.