Antioxidant, and Antiviral Activities of Tyrosol (TY), Hydroxytyrosol (HTY) and their Esters

A series of lipophilic esters of tyrosol and hydroxytyrosol, two naturally occurring phenols in olive oil with interesting biological properties, were prepared by an enzymatic and saponification proceduret. The antioxidant activities of esters so produced were evaluated using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay, human low-density lipoprotein (LDL) oxidation assay, and DNA strand scission assay. The antiviral properties of selected esters were measured by hepatitis C virus (HCV) protease inhibitory and alpha-glucosidase inhibitory activity. All methods used for the antioxidant activity evaluation indicated that tyrosol (TY) and its derivatives were less active than the corresponding hydroxytyrosols (HTY), which reflected the influence of the ortho-diphenolic (catechol) structure of the latter on antioxidant capacity. For hydroxytyrosol, the introduction of the lipid part decreased its antioxidant activity. We observed the inhibition of HTY saturated fatty acid esters against hydroxyl radical induced DNA oxidation decreased as alkyl chain length increased. Meanwhile, an unsaturation-dependent antioxidant effect was observed for TY and HTY esters in DNA strand scission assay, and for TYr esters in DPPH assay. However, in the LDL oxidation assay, the polyunsaturated fatty acid moiety of TYr esters may be oxidized. For antiviral properties of selected esters, most of the TY derivatives that showed potent inhibition on α-glucosidase were not active against HCV while HTY esters showed very good HCV protease inhibition, especially HTY caprylate, stearate and oleate ester which displayed 3-fold stronger inhibition than that of embelin (the positive control).