5β Reductase

5β-Reductase, or Δ⁴-3-oxosteroid 5β-reductase (EC 1.3.1.3, 3-oxo-Δ⁴-steroid 5β-reductase, androstenedione 5β-reductase, cholestenone 5β-reductase, cortisone 5β-reductase, cortisone Δ⁴-5β-reductase, steroid 5β-reductase, testosterone 5β-reductase, Δ⁴-3-ketosteroid 5β-reductase, Δ⁴-5β-reductase, Δ⁴-hydrogenase, 4,5β-dihydrocortisone:NADP⁺ Δ⁴-oxidoreductase, 3-oxo-5β-steroid:NADP⁺ Δ⁴-oxidoreductase) is an enzyme with systematic name 5β-cholestan-3-one:NADP⁺ 4,5-oxidoreductase. This enzyme catalyses the following chemical reaction

(1) 5β-cholestan-3-one + NADP⁺ ⇌ cholest-4-en-3-one + NADPH + H⁺(2) 17,21-dihydroxy-5β-pregnane-3,11,20-trione + NADP⁺ ⇌ cortisone + NADPH + H⁺

The human enzyme efficiently acts on progesterone, 17α-hydroxyprogesterone, androstenedione, and testosterone to 5β-reduced metabolites. It can also act on aldosterone, corticosterone and cortisol, to some extent.