Inhibition of protein radical reactions of ferrylmyoglobin by the water-soluble analog of vitamin E, Trolox C.

The reactivity of Trolox C, a water-soluble analog of vitamin E, toward protein radicals formed during the oxidation of metmyoglobin and the therefrom derived molecular products was examined in terms of the ability of the phenolic antioxidant to prevent specific oxidative reactions involving tyrosyl radicals and to reduce the molecular products to functional hemoproteins. (i) Trolox prevented the oxidative reactions initiated upon oxidation of the hemoprotein by H2O2 and involving tyrosyl radicals: on one hand, it inhibited the covalent binding of protein to the heme group and, on the other hand, it inhibited dimerization of sperm whale myoglobin, the latter process entailing the intermolecular covalent binding of tyrosines. The inhibition of both processes required concentrations of Trolox 20- to 50-fold higher than those needed to reduce the hypervalent heme iron to its ferric form. Likewise, Trolox inhibited dimerization and polymerization of sperm whale myoglobin upon its treatment with chloroiridate, a process associated with formation of tyrosyl radicals and negligible oxidation of the heme iron. However, these results did not provide unambiguous evidence for the reactivity of the phenolic antioxidant toward amino acid radicals in myoglobin, for Trolox displayed a high reactivity toward the oxidant, chloroiridate. (ii) The stable oxidation product originating from sperm whale myoglobin oxidation, i.e., the dimeric hemoprotein, was redox active, inasmuch as it retained its capacity to form an oxy complex upon reduction and an oxoferryl complex upon oxidation by H2O2. The latter complex was reduced by Trolox to a compound which exhibited a 10- to 12-nm blue shift of the 632-nm absorption typical of the native metmyoglobin. Although this absorption shift was likely to express oxidative modifications of the porphyrin ring, the modified hemoprotein retained its capacity to react with peroxides and displayed a peroxidative activity.