Antioxidant and biological activities of tyrosol, hydroxytyrosol and their esters

Sun, Yuanxin (2017) Antioxidant and biological activities of tyrosol, hydroxytyrosol and their esters. Masters thesis, Memorial University of Newfoundland.

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Abstract

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 simplified base extraction procedure. Both tyrosol and hydroxytyrosol are hydrophilic molecules with poor solubility in lipophilic media, resulting in limited usage in foods and limited uptake by the cells and bioavailability in vivo. The antioxidant activities of esters so produced were evaluated using the 2,2-diphenyl-1-picrylhydrazyl (DPPH), human low-density lipoprotein (LDL) oxidation, and DNA strand scission assays. The antiviral properties of selected esters were measured using hepatitis C virus (HCV) protease and alpha-glucosidase inhibitory activities. Antiglycation by bovine serum albumin (BSA)-glucose assay, protection effect against oxidative stress, generation anti-inflammatory products by nitrite assay, and cytotoxic properties by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay of selected esters were also evaluated. 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 moiety 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 TY esters in DPPH assay. However, in the LDL oxidation assay, the polyunsaturated fatty acid (PUFA) moiety of TY 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 esters which displayed 3-fold stronger inhibition than that of embelin (the positive control). Moreover, it was found that lipophilization by esterification could improve the anti-inflammatory and antiglycation effects of tyrosol and hydroxytyrosol. These results indicate that the lipophilic ester derivatives can served as antioxidant ingredients in food, as well as anti-inflammatory, antiglycation, and antiviral agents, and ingredients in other therapeutic applications, supplements and natural health products.

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