Tyrosol related compounds

Oleuropein and its metabolites are terpenoids present in all parts of plant and represent the chemotaxonomic markers of O. europaea. Tyrosol, hydroxy-tyrosol and related compounds represent the main characteristic phenolic fraction of O. europaea and are recognised as very powerful anti-oxidising agents. 

Hydroxytyrosol potently inhibited the synthesis of leukotriene B4 and its metabolites 20-hydroxy and 20-carboxy LTB4 in calcium-ionophore stimulated human PMN leukocytes. IC50 was 1.2 pM zileuton used as reference compound had an IC50 of 1.5 pM [81]. Inhibition of 5- and 12-lipoxygenases from rat platelets and leukocytes by hydroxytyrosol was shown by Kohyama et al [79]. These authors compaired the inhibition by hydroxytyrosol with other related phenols present in olive oil (tyrosol. 

The various classes of minor constituents can be divided into two groups. The first group consists of fatty acid derivatives such as mono- and diacylglycerols, phospholipids, waxes and esters of sterols. The second group includes classes of compounds not related chemically to fatty acids hydrocarbons, aliphatic alcohols, free sterols, tocopherols, chlorophylls, carotenoids and polar compounds such as tyrosol and hydroxytyrosol. 

In the olive, there are two main compounds, besides that reported in the literature. The first one is oleuropein, the first secoiridoid isolated in 1960 by Panizzi in Roma, as mentioned above. The second one is the comoside that is a hemiquinone glucoside, structurally and biogenetically related to hydroxy-tyrosol that is the principal free phenol in the olive. It should be remembered that, depending on the olive cultivar, the oleuropein can be prevalent against the comoside, or it could be the contrary, until reaching the equality. 

Pharmacological studies were mainly focussed on oleuropein that represents die key phenolic compound in O. europaea, and on related phenolic compounds such as tyrosol and hydroxy-tyrosol. Therefore, the following discussion will be focussed on reported biological activity of these compounds. 

The resistence to oxidation was related to the high content of polyphenols removal of polyphenols greatly reduced oxidative stability [39]. The subsequent studies were devoted to identifying the compounds which more contribute to oil stability and the mechanism of protection. Hydroxytyrosol and caffeic acid were better protection factors than BHT tyrosol and other monohydroxy phenols gave little contribution to oil stability [43] oil shelf-life was positively correlated to hydroxytyrosol / tyrosol ratio [44]. 

Studies performed in vitro also demonstrated the unique antioxidant properties of virgin olive oil phenohcs against LDL oxidation [181-183]. These effects could be related to the ability of olive oil phenolics, such as tyrosol, to bind LDL in vitro and thus to protect LDL and other phenolic compounds previously bound to LDL from oxidation [182,183]. 

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