In enzymology, 3-keto-steroid reductase () is an enzyme that catalyzes the reduction of keto sterols to the corresponding 3ò-hydroxysteroids. For example:
In this example, the two substrates of the enzyme are lophenol and oxidised nicotinamide adenine dinucleotide phosphate (NADP<sup>+</sup>). Its products are 4ñ-methyllathosterone, reduced NADPH, and a proton.
This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD<sup>+</sup> or NADP<sup>+</sup> as acceptor. The systematic name of this enzyme class is 3beta-hydroxy-steroid:NADP<sup>+</sup> 3-oxidoreductase. This enzyme is also called 3-KSR. This enzyme participates in biosynthesis of steroids.