Reduction of NAD + and NADP + and reductive cleavage of NADH and NADPH yielding NAD . and NADP .

Abstract

The reductive cleavage of NADH (and/or NADPH) was studied by means of controlled-potential electrolysis at mercury and platinum cathodes. The reduction products were characterized using high performance liquid chromatography (HPLC). NAD 2 dimers were found (30% of NADH converted) after electrolysis at the platinum electrode at −1.0 V (vs. SCE) in a buffered medium. Chemisorbed NAD . resulted after the reduction of 0.5–2.0 mmol/1 NADH on the mercury electrode. Homogeneous reduction of NADH with dithionite yielded NAD + after air oxidation of the original products. In order to identify the responses of NAD ., the reversibility of the redox reaction NAD +/NAD . (and NADP +/ NADP .) was studied by means of reverse pulse polarography (RPP) in a buffered neutral medium. The large potential difference between NAD + reduction (at approximately −1.0 V) and NAD . oxidation was explained by the chemisorption of NAD . at the mercury electrode. Reductive cleavage of the C(4)-H bond in the dihydronicotinamide moiety of NADH occurred under conditions identical to those of NAD + reduction. In the coenzyme action, the same electron-transfer reaction to NAD + or to NADH could produce either a hydrogen atom acceptor NAD . (from NAD + ) or a H . donor from NADH.