The susceptibility of low-density lipoprotein (LDL) to oxidation was studied in hypertriglyceridemic men (5 with type III and 5 with type IV) at baseline on a low-saturated-fat, low-cholesterol diet, after 6 weeks of dietary supplementation with fish oil (Promega, 12 g/d), and after 6 weeks of fish oil combined with probucol (500 mg BID). The relative content of n-3 polyunsaturated fatty acids in plasma and LDL was increased during the two treatment periods, and a low a-tocopherol to n-3 polyunsaturated fatty acids ratio was observed. Plasma thiobarbituric acid-reactive substances (TBARS) levels were unchanged after 6 weeks of fish oil, but the ratio of lipid peroxides to the reduced triglyceride (TG) levels (MDA: TG) was significantly higher (P<.01). Addition of probucol lowered both absolute levels of TBARS (P<.01) and the MDA to TG ratio (P<.001). The susceptibility of LDL to Cu 2+ -catalyzed oxidation was evaluated over a 5-hour time course by determining TBARS formation, free amino group levels, and changes in LDL electrophoretic mobility. TBARS levels that were higher in native LDL (1.019«/< 1.050 g/mL) after 6 weeks offish oil than at baseline (P<.0l) were reduced 52.3±11.3% by the addition of probucol (P<.001). With fish oil alone, TBARS production after exposure of LDL to Cu

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