Renalase is a recently identified flavoprotein that has been associated with numerous physiological maladies. There remains a prevailing belief that renalase functions as a hormone, imparting an influence on vascular tone and heart rate by oxidizing circulating catecholamines, chiefly epinephrine. This activity, however, has not been convincingly demonstrated in vitro, nor has the stoichiometry of this transformation been shown. In prior work we demonstrated that renalase induced rapid oxidation of low-level contaminants of β-NAD(P)H solutions ( Beaupre, B. A. et al. (2013) Biochemistry 52 , 8929 - 8937 ; Beaupre, B. A. et al. (2013) J. Am. Chem. Soc . 135 , 13980 - 13987 ). Slow aqueous speciation of β-NAD(P)H resulted in the production of renalase substrate molecules whose spectrophotometric characteristics and equilibrium fractional accumulation closely matched those reported for α-anomers of NAD(P)H. The fleeting nature of these substrates precluded structural assignment. Here we structurally assign and identify two substrates for renalase. These molecules are 2- and 6-dihydroNAD(P), isomeric forms of β-NAD(P)H that arise either by nonspecific reduction of β-NAD(P)(+) or by tautomerization of β-NAD(P)H (4-dihydroNAD(P)). The pure preparations of these molecules induce rapid reduction of the renalase flavin cofactor (230 s(-1) for 6-dihydroNAD, 850 s(-1) for 2-dihydroNAD) but bind only a few fold more tightly than β-NADH. We also show that 2- and 6-dihydroNAD(P) are potent inhibitors of primary metabolism dehydrogenases and therefore conclude that the metabolic function of renalase is to oxidize these isomeric NAD(P)H molecules to β-NAD(P)(+), eliminating the threat they pose to normal respiratory activity.