Novel kinetics in a biomimetic redox reaction involving NADH and tetrazolium salts in aqueous micellar solutions

Aqueous micelles of Triton X-100 are shown to catalyze the redox reaction between NADH and 2-p-iodophenyl-3-p-nitrophenyl-5-phenyltetrazolum chloride (INT) at neutral pH. The reaction exhibits a first-order dependence on NADH when INT is saturating; conversely, when NADH is saturating, the dependence is strictly second-order with respect to INT. The second-order dependence of the reaction on INT is also evident in situations where micelles of a cationic detergent are used in place of Triton X-100. The available kinetic evidence indicates the transient formation of a central complex involving the addition of two molecules of INT and one molecule of NADH to a "site" on the micelle where they are held together until completion of the redox process. However, the reaction does not seem to proceed by successive 1-e- steps, suggesting that the second-order dependence on INT has no bearing on the mechanism of redox process. The transfer of reducing equivalents between NADH and INT is shown to be direct and quantitative, with the redox steps confined to a microenvironment, as in the case of enzymatic NAD(P)H-dependent reactions. A mechanism consistent with the hydridic nature of the migrating hydrogen from the C(4) position of the dihydropyridine nucleus of NADH is proposed, assuming that only one molecule of INT in the central complex participates in the actual redox process and that the other molecule of INT acts as a cocatalyst by way of providing the necessary "basicity" at the reaction site.