Inhibition of photooxidation of alpha-tocopherol by quercetin in human blood cell membranes in the presence of hematoporphyrin as a photosensitizer.

Photooxidation of alpha-tocopherol (alpha-TH) and photoperoxidation of lipids in blood cell membranes in the presence of hematoporphyrin (HP) as a photosensitizer were inhibited by quercetin. Half maximal inhibition for the photooxidation of alpha-TH was obtained at about 0.3 mM quercetin and that for the lipid photoperoxidation at about 1.5 microM quercetin. The difference of the half maximal inhibition may be due to the difference of mechanism of the inhibition between the two reactions. O2- and H2O2 hardly participated in the photooxidation of alpha-TH and 1O2 participated in the photooxidation only partially (about 5%). The electron transfer reaction from alpha-TH to excited HP was indicated by measuring ferricyanide photoreduction in the suspensions of alpha-TH in PBS solution in the presence of HP. The photooxidation of alpha-TH in PBS solution was inhibited by quercetin and vice versa. In the presence of linoleic acid in PBS solution, quercetin inhibited the photooxidation of alpha-TH and alpha-TH stimulated the photooxidation of quercetin. Based on the above data, as possible mechanisms of the inhibition of photooxidation of alpha-TH in blood cell membranes by quercetin, competition of quercetin with alpha-TH for excited HP and for radicals generated during lipid peroxidation and reduction of oxidized alpha-TH by quercetin are proposed. The antioxidative function of quercetin was enhanced by ascorbate even under conditions in which ascorbate functioned as a prooxidant when it was added alone. The enhancement is attributed to the functions of ascorbate to reduce the oxidized quercetin and of quercetin to inhibit ascorbate photooxidation.