CHOLESTERYL ESTER HYDROPEROXIDE FORMATION IN MYOGLOBIN-CATALYZED LOW-DENSITY-LIPOPROTEIN OXIDATION - CONCERTED ANTIOXIDANT ACTIVITY OF CAFFEIC AND P-COUMARIC ACIDS WITH ASCORBATE

Two diet-derived phenolic acids, caffeic and p-coumaric acids, interpl ayed with ascorbate in the protection of low density lipoproteins (LDL ) from oxidation promoted by ferrylmyoglobin. Ferrylmyoglobin, a two-e lectron oxidation product from the reaction of metmyoglobin and H2O2, was able to oxidize LDL, degrading free cholesterol and cholesteryl es ters. Upon exposure to, ferrylmyoglobin, LDL became rapidly depleted o f cholesteryl arachidonate and linoleate, which turn into the correspo nding hydroperoxides. Cholesteryl oleate and cholesterol were, compara tively, more resistant to oxidation. Caffeic (2 mu M) and p-coumaric ( 12 mu M) acids efficiently delayed oxidations, as reflected by an incr ease in the lag times required for linoleate hydroperoxide and 7-ketoc holesterol formation as well as for cholesteryl linoleate consumption. At the Same concentration, ascorbate, a standard water-soluble antiox idant, was less efficient than the phenolic acids. Additionally, pheno lic acids afforded a protection to LDL that, conversely to ascorbate, extends along the time, as inferred from the high levels of cholestery l linoleate and cholesteryl arachidonate left after 22 hr of oxidation challenging. Significantly, the coincubation of LDL viith ascorbate a nd each of the phenolic acids resulted in a synergistic protection fro m oxidation. This was inferred from the lag phases of cholesteryl lino leate hydroperoxide (the major peroxide found in LDL) formation in the presence of mixtures of ascorbate with phenolic acids longer than the sum of individual lag phases of ascorbate and the phenolic acids. A s imilar description could be drawn for the accumulation of a late produ ct of oxidation, 7-ketocholesterol. It is concluded that ferrylmyoglob in induces a typical pattern of LDL lipid peroxidation, the oxidation rate of cholesteryl esters being a function of unsaturation; furthermo re, there is a synergistic antioxidant activity of diet-derived phenol ic acids with ascorbate in the protection of LDL from oxidation, a fin ding of putative physiological relevance. (C) 1998 Elsevier Science In c.