IN-VITRO EVALUATION OF THE ANTIOXIDANT ACTIVITY AND BIOMEMBRANE INTERACTION OF THE PLANT PHENOLS OLEUROPEIN AND HYDROXYTYROSOL

Oleuropein and hydroxytyrosol, two phenolic compounds contained in oli ves and olive oil, are known to possess several biological properties, many of which may be related, partially at least, to their antioxidan t and free radical-scavenger ability. Hence, together with their scave nging activity against the stable 1,1-diphenyl-2-picrylhydrazyl radica l (DPPH test), we have investigated the antioxidative effect of oleuro pein and hydroxytyrosol in a model system consisting of dipalmitoylpho sphatidylcholine/linoleic acid unilamellar vesicles (DPPC/LA LUVs) and a water-soluble azo compound as a free radical generator (LP-LUV test ). The results obtained were also interpreted in the light of biopheno l interactions, studied by differential scanning calorimetry (DSC), wi th dimyristoylphosphatidylcholine (DMPC) vesicles as a biological memb rane model. Our results obtained in the DPPH and LP-LUV tests confirm the good scavenger activity and antioxidant effect of oleuropein and h ydroxytyrosol. However, while both compounds exhibit comparable effect iveness in the DPPH test (hydroxytyrosol being slightly more active th an oleuropein), oleuropein seems, in the LP-LUV test, a better antioxi dant than hydroxytyrosol. Besides oleuropein shows a better antioxidan t activity in the membranous system than in homogenous solution. Furth ermore, oleuropein, but not hydroxytyrosol, interacts with DMPC vesicl es, causing shifts, toward lower values, of the calorimetric peak temp erature (T-m), associated to the gel to liquid-crystal phase transitio n, typical for DMPC multilayers. The hypothesis will be discussed that hydroxytyrosol can serve as scavenger of aqueous peroxyl radicals nea r the membrane surface, while oleuropein acts also as a scavenger of c hain-propagating lipid peroxyl radicals within the membranes. (C) 1998 Elsevier Science B.V. All rights reserved.