Radiolabelling studies of free radical reactions using muonium (the second hydrogen radioisotope): Evidence of a direct antioxidant role for vitamin K in repair of oxidative damage to lipids

Following the recent suggestion that oxidative damage to lipids might involve the formation of C-2 glyceryl radicals, in addition to main-chain allylic radicals, which are generally thought important, we studied the kinetics of potential 'repair' reactions by vitamin E and vitamin K₁ (which are lipophilic and tend to locate in cell membranes) with 1,1,2-trimethylallyl and 1-acetoxyprop-2-yl radicals as models of, respectively, main-chain and glyceryl-type radicals. The second-order rate constants for the reaction between trimethylallyl radicals and vitamin E and vitamin K₁ are 9.38 × 10⁶ and 3.54 × 10⁸1mol^-1s^-1 and those between 1-acetoxyprop-2-yl radicals and vitamin E and vitamin K₁ are 1.76 × 10⁸ and 2.03 × 10⁶1mol^-1s^-1, respectively. The results suggest that direct scavenging of glyceryl radicals by vitamin E should be an efficient process, and of main-chain allylic radicals fairly so. Additionally, it appears that vitamin K₁ can act directly as a 'radical repair agent,' given its relatively high reactivity with allylic radicals (four times faster than with vitamin E), so prior reduction to the quinol form may be unnecessary.