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Steroid 5α-reductase reduces the conjugated double bond of particular steroids using NADPH. Human type 2 can convert testosterone to dihydrotestosterone and generate an inhibitory cofactor adduct from finasteride.

Finasteride is an unsaturated 4-azasteroid inhibitor of steroid 5α-reductase. Enzyme-generated reduction and enolate capture of oxidized cofactor give a covalent cofactor–inhibitor adduct; the protein itself need not be covalently modified.

catalysis requires a protonated group, consistent with a general-acid contribution. Loss of its proton as pH rises would remove carbonyl activation or the proton needed to finish the conjugate reduction. A glutamate residue with a shifted active-site $p{K}_a$ is a plausible candidate, but a pH cutoff alone neither identifies the residue nor measures a precise $p{K}_a$. Irreversible unfolding or changes in substrate binding must also be excluded by reversible pH jumps, protein-integrity checks and initial-rate measurements.

For the mechanistic notation below, the reduced cofactor is written NADPH, the established cofactor of human steroid 5α-reductase. The original PDF explicitly labels it NADH; its abbreviated N-substituent does not display the phosphate distinction. NADH and NADPH have the same reactive reduced nicotinamide ring, so the local hydride-transfer arrows are unchanged, although their biochemical specificity is different.^[ref]

Testosterone is a steroid with a conjugated C3-ketone/C4=C5 fragment. 5α-reductase adds hydride at C5 and a proton at C4 to make dihydrotestosterone.