Modulation of human platelet function by food flavonoids.

Flavonoids, a group of polyphenolic natural compounds, represent an important source of antioxidants in the human diet [ 1,2]. Although the mechanism of action of flavonoids is not well understood, these compounds have been found to be inversely related to mortality from coronary heart disease[3]. Aggregation of platelet is fundamental to a wide range of physiological processes, i.e. in the events ranging from normal blood coagulation to the extremes of thrombosis and atherosclerosis [4,5], therefore the aim of this study was to investigate the effects of several dietary flavonoids (catechin, myricetin, querectin, apigenin and morin) and a-tocopherol on platelet function. Platelet iich plasma (PRF') was prepared from blood collected in sodium citrate (10% v/v of 3.8% citrate solution) from healthy volunteers who had not taken any medication for at least 14 days prior to phlebotomy. Platelets were counted in PRP and adjusted to a constant platelet count (2.5x108/ml) with platelet poor-plasma (PPP), obtained by further centrifugation of the blood remaining after removal of PRP. Washed platelets (WP) were prepared from PRP as described [6]. Aggregation of platelets was induced by either collagen or ADP. Maximal amplitude of aggregation of PRP was obtained with lOpg/ml collagen and 10 pM ADP, whereas WP were aggregated with collagen only. The inhibitory effects of flavonoids and vitamin E were initially evaluated at different times of preincubation either with PRP or WP with the compound dissolved in ethano1:water at a final ethanol concentration of 0.13%. Control samples were preincubated with vehicle alone. Aggregatory responses in the absence or presence of these compounds were evaluated by measuring the amplitude curves with different concentrations of these compounds in three separate experiments using platelets from different subjects. Plasma levels of fibrinogen, vitamin E and vitamin C of these volunteers were determined and were found to be within the normal range. Apigenin, quercetin, morin and a-tocopherol did not inhibit platelet aggregation in PRP induced by collagen, ADP, or arachidonic acid (AA) (n=5). Conversely, catechin inhibited collagen-induced aggregation by approximately 4.3% (n=7) whereas myricetin inhibited AA-induced aggregation by 75%, ADP-induced aggregation by 21 % and collagen-induced aggregation by 5% in PRP (n=2). In contrast to PRP, aggregation of WP was significantly inhibited by these flavonoids except apigenin and a-tocopherol (Table). Minimum concentration of flavonoids required to inhibit 50% platelet aggregation (Icso) induced by collagen was determined for these flavonoids. The IC50 value for myricetin was 57f6 pM whereas for morin and catechin the values were very high, 323f78 pM and 591f 65 @4 (n=3), respectively. Table: % inhibition of platelet aggregation by various flavonoids tn WP