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Significance of the Zero-Current Potential of the NAD+ - NADH System

Abstract : Since the first electrochemical reduction step of NAD+ and the elec­trochemical oxidation step of NADH lie respectively at -0.69 and 0.5­ - 0.9 V (N.H.E.), no direct measure of the formal potential of NAD+ ~­ NADH system may be attained potentiometrically. We examine the sig­nificance of the previous potentiometric studies realized in the pres­ence of a mediator and an enzyme. A reliable zero-current potential of NAD+ and NADH solutions is only obtained when small amounts of benzyl-viologen (BV2+) and xanthine oxidase (X0) are added. The var­ious electrochemical reduction steps of 10 to 1000 µM BV2+ aqueous solutions are studied at pH 9.8 on dropping mercury and rotating plati­num disk electrodes. The formal potential of the BV2+ - BV.+ system, equal to -0.360 V (N.H.E.), may be measured on a platinum electrode, while on a mercury electrode a strong adsorption interferes and gives a polar­ographic prewave. When XO is present, the addition of NADH decreases the BV2+ reduction waves: an anodic wave corresponding to BV.+ oxi­dation appears at the same potential as the first BV2+ reduction wave. The zero-current potential measured in NAD+, NADH, BV2+, BV.+ and XO solutions is actually fixed by the BV2+-BV.+ system, which equilibrates through chemical oxido-reduction reactions with the NAD+ ~­ NADH system.
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Philippe Leduc, Daniel Thevenot, René Buvet. Significance of the Zero-Current Potential of the NAD+ - NADH System. Bioelectrochemistry and Bioenergetics, Elsevier, 1976, 3, pp.491 - 508. ⟨hal-01082731⟩

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