Polyphenols bind to low density lipoprotein at biologically relevant concentrations that are protective for heart...

There is ample evidence in the epidemiological literature that polyphenols, the major non-vitamin antioxidants in plant foods and beverages, have a beneficial effect on heart disease. Until recently other mechanisms which polyphenols exhibit such as cell signaling and regulating nitric oxide bioavailability have been investigated. The oxidation theory of atherosclerosis implicates LDL oxidation as the beginning step in this process. Nine polyphenols from eight different classes and several of their O-methylether, O-glucuronide and O-sulfate metabolites have been shown in this study to bind to the lipoproteins and protect them from oxidation at lysosomal/inflammatory pH (5.2), and physiological pH (7.4). Polyphenols bind to the apoprotein at pH 7.4 with K_b > 10⁶ M^-1 and the number of molecules of polyphenols bound per LDL particle under saturation conditions varied from 0.4 for ferulic acid to 13.1 for quercetin. Competition studies between serum albumin and LDL show that substantial lipoprotein binding occurs even in the presence of a great molar excess of albumin, the major blood protein. These in vitro results are borne out by published human supplementation studies showing that polyphenol metabolites from red wine, olive oil and coffee are found in LDL even after an overnight fast. A single human supplementation with various fruit juices, coffee and tea also produced an ex vivo protection against lipoprotein oxidation under postprandial conditions. This in vivo binding is heart-protective based on published olive oil consumption studies. Relevant to heart disease, we hypothesize that the binding of polyphenols and metabolites to LDL functions as a transport mechanism to carry these antioxidants to the arterial intima, and into endothelial cells and macrophages. Extracellular and intracellular polyphenols and their metabolites are heart-protective by many mechanisms and can also function as potent "intraparticle" and intracellular antioxidants due to their localized concentrations that can reach as high as the micromolar level. Low plasma concentrations make polyphenols and their metabolites poor plasma antioxidants but their concentration in particles such as lipoproteins and cells is high enough for polyphenols to provide cardiovascular protection by direct antioxidant effects and by other mechanisms such as cell signaling.