Dehydrogenases form a class of enzymes that catalyze reversible oxidation-reduction reactions and are largely dependent on nicotinamide adenine dinucleotide coenzymes: NAD(H) or NADP(H).

Hydride transfer to and from the pro-chiral carbon at the pyridine C4-position of the dihydronicotinamide ring of NAD or NADP is known to be stereospecific. A-stereospecific enzymes transfer from the A-side of the ring (the pro-R hydrogen) and B-stereospecific enzymes transfer from the B-side of the ring (the pro-S hydrogen). Dehydrogenase stereoselectivity for one of the two diastereotopic hydrogens at the C4 position is known to be absolute, unlike stereospecificity for substrate. The two hydrogens at the C4 position of NAD(P) are diastereotopic because there are other stereocenters in the molecule.

To date, the number of dehydrogenases showing pro-R specificity is roughly equivalent to the number showing pro-S specificity. No known dehydrogenase is stereorandom. Observed stereospecificity is dependent on the structure of the NAD(P) binding site. Depending on how the dihydronicotinamide ring is bound in the active site, either the pro-R or pro-S hydrogen will be positioned towards the substrate.