Transient species in the stepwise interconversion of NADH and NAD+

Two mechanisms of the interconversion of NADH and NAD(+) in the coenzyme itself and in its analogues are discussed: a one-step hydride transfer and a stepwise electron-proton-electron transfer. Direct characterization of the transient species in the stepwise process and inversion of the stability order of the keto and enol tautomers is presented. The nonenzymatic and enzyme-mediated reactions of selected pyridinium salts that affect the NADH reversible arrow NAD(+) equilibrium are discussed in terms of their potential cytotoxicity.