Catalyzes the epimerization of the S- and R-forms of NAD(P)HX, a damaged form of NAD(P)H that is a result of enzymatic or heat-dependent hydration. This is a prerequisite for the S-specific NAD(P)H-hydrate dehydratase to allow the repair of both epimers of NAD(P)HX. Accelerates cholesterol efflux from endothelial cells to high-density lipoprotein (HDL) and thereby regulates angiogenesis (By similarity). (6R)-NADHX = (6S)-NADHX (6R)-NADPHX = (6S)-NADPHX Binds 1 potassium ion per subunit. Homodimer (By similarity). Interacts with APOA1 and APOA2 (By similarity). In sperm, secretion gradually increases during capacitation. Undergoes physiological phosphorylation during sperm capacitation, downstream to PKA activation. Belongs to the NnrE/AIBP family.