Enzyme converting testosterone into DHT:
Development of male external genitalia requires local conversion of testosterone secreted by Leydig cells into more active androgen dihydrotestosterone. Reaction catalyzed by microsomal enzyme steroid 5α-reductase type 2 encoded by SRD5A2 gene which uses NADPH to reduce Δ4-5 double bond. DHT exhibits higher affinity for androgen receptor and prolonged receptor-DNA interaction, activating genes for penile growth, urethral closure and scrotal fusion. Deficiency of enzyme causes pseudovaginal perineoscrotal hypospadias despite normal Wolffian ducts, while aromatase converts testosterone to estrad
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 17: 5α-reductase converts testosterone to DHT.