Protective activity of hydroxytyrosol metabolites on erythrocyte oxidative-induced hemolysis

The capacity of important hydroxytyrosol metabolites (homovanillyl alcohol, hydroxytyrosol acetate, homovanillyl alcohol acetate, hydroxytyrosol 3′ and 4′-O-glucuronides, and homovanillyl alcohol 4′-O-glucuronide) to protect red blood cells (RBCs) from oxidative injury induced by the radical initiator 2,2′-azo-bis(2-amidinopropane) dihydrochloride (AAPH) or by the natural radical initiator H₂O₂ was evaluated. In the presence of AAPH, all compounds showed to protect RBCs from hemolysis in a dose-dependent manner, exccept for the homovanillyl alcohol glucuronide, with the order of activity being at 20 μM hydroxytyrosol > hydroxytyrosol glucuronides = hydroxytyrosol acetate = homovanillyl alcohol = homovanillyl acetate > homovanillyl alcohol glucuronide. At 10 μM, hydroxytyrosol, hydroxytyrosol acetate, and hydroxytyrosol glucuronides still protected hemoglobine from oxidation and from morphological RBC changes. In the presence of H₂O₂, hydroxytyrosol showed to significantly protect RBCs from oxidative hemolysis in a dose-dependent manner, but the hydroxytyrosol glucuronides showed only a limited protection that was independent of the concentration used.