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