Abstract
A kinetic study of the regeneration reactions of natural vitamin E by vitamin C and ubiquinol was carried out by means of double-mixing stopped-flow spectroscopy. A substantial deuterium kinetic-isotope effect was observed on the second-order rate constant and the activation energy for ubiquinol. In the regeneration reaction of α-tocopherol, deuteration of ubiquinol increased and decreased the activation energy and the second-order rate constant by 6.1 kJ/mol and a factor of 18.3, respectively. From this result, it is considered that proton tunneling plays an important role in the regeneration reaction of vitamin E by ubiquinol. In contrast, since such a substantial deuterium kinetic isotope effect was not found in the regeneration reaction of α-tocopherol by vitamin C, the tunneling effect may not play an important role under the present experimental conditions. However, for 5,7-diisopropyltocopherol a substantial deuterium kinetic isotope effect was observed. The conditions under which the tunneling effect becomes an important factor were discussed in conjunction of our experimental results. Vitamin E may inhibit the autooxidation of lipids in cellular membranes by taking advantage of proton tunneling. It is interesting that the microscopic quantum-mechanical tunneling effect could manifest itself in a macroscopic vital function. Although this is in conflict with our intuition at first glance, living cells might, in reality, know quantum mechanics very well and use it effectively.
