FUNCTION Plays a key role in early steps of protein N-linked glycosylation by being involved in the conversion of polyprenol into dolichol (PubMed:20637498, PubMed:38821050). Acts as a polyprenal reductase that mediates the reduction of polyprenal into dolichal in a NADP-dependent mechanism (PubMed:38821050). Dolichols are required for the synthesis of dolichol-linked monosaccharides and the oligosaccharide precursor used for N-glycosylation (PubMed:20637498, PubMed:38821050). Also able to convert testosterone (T) into 5-alpha-dihydrotestosterone (DHT) (PubMed:17986282, PubMed:26855069).CATALYTIC ACTIVITY a di-trans,poly-cis-dolichal + NADP(+) = a di-trans,poly-cis-polyprenal + NADPH + H(+)CATALYTIC ACTIVITY a 3-oxo-5alpha-steroid + NADP(+) = a 3-oxo-Delta(4)-steroid + NADPH + H(+)CATALYTIC ACTIVITY androst-4-ene-3,17-dione + NADPH + H(+) = 5alpha-androstan-3,17-dione + NADP(+)CATALYTIC ACTIVITY 17beta-hydroxy-5alpha-androstan-3-one + NADP(+) = testosterone + NADPH + H(+)PATHWAY Protein modification; protein glycosylation.SUBCELLULAR LOCATION Expressed in preadipocytes (at protein level) (PubMed:26855069). Overexpressed in hormone-refractory prostate cancers (HRPC). Almost no or little expression in normal adult organs.DISEASE The disease is caused by variants affecting the gene represented in this entry.DISEASE The disease is caused by variants affecting the gene represented in this entry.SIMILARITY Belongs to the steroid 5-alpha reductase family. Polyprenal reductase subfamily.CAUTION Was initially characterized as a polyprenol reductase, mediating the conversion of polyprenol into dolichol (PubMed:20637498). However, it was later shown to catalyze an intermediate step in this pathway and reduce polyprenal (PubMed:38821050).