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). {ECO:0000269|PubMed:17986282, ECO:0000269|PubMed:20637498, ECO:0000269|PubMed:26855069, ECO:0000269|PubMed:38821050}. This is the function of SRD5A3 (steroid 5 alpha-reductase 3, ENSG00000128039).