Background and Aim: We have previously shown that LDL enrichment with beta-carotene either in vitro or in vivo reduced the susceptibility of LDL to oxidation, whereas some other studies failed to show a similar effect. In the present study we examined the antioxidative effects against LDL oxidation of the two major LDL-associated carotenoids: lycopene and beta-carotene. Methods and Results: The study was performed on human LDL obtained from 22 healthy subjects. All LDL samples were supplemented in vitro with 3 mu M of either tomato lycopene or beta-carotene, after which they were exposed to metal ion-dependent (CuSO4) or -independent [2,2-azobis, 2,4-dimethyl-valeronitril (AMVN)] oxidation. Seven LDLs, out of the 12 samples, responded to their enrichment with carotenoids by reduced susceptibility to oxidation ("responder" LDLs), whereas in five LDL samples there was no effect of LDL enrichment with carotenoids on LDL oxidizability ("non-responder" LDLs). In the "responder" LDLs, supplementation of the LDL with tomato lycopene or with beta-carotene, resulted in a significant inhibition of LDL oxidation when induced by CuSO4 (by 65% and 37% respectively), by the free radical generators, AMVN (by 68% and 52% respectively), or by J-774 A.1 macrophages (by 52% and 48% respectively). Comparison of the antioxidant status in the "responder" LDLs, to the "non-responder" LDLs, revealed that their vitamin E content was significantly (p<0.01) higher. We thus analyzed the effect of the carotenoids in combination with vitamin E, on the susceptibility of LDL to copper ion-induced oxidation. A synergistic antioxidative effect against LDL oxidation was obtained when a combination of the carotenoids together with vitamin E was used instead of using the individual antioxidant separately Since natural antioxidants exist in nature in combination, we studied the effect of tomato oleoresin (the lipid extract of tomato which contains lycopene, vitamin E and beta- carotene) on LDL oxidation. In vitro supplementation of human LDLs with tomato oleoresin resulted in a remarkable dose-dependent inhibition of copper ion-induced LDL oxidation, as shown by the prolongation of the lag phase, and by a reduction in the lipoprotein-associated thiobarbituric acid reactive substances, by up to 90%. To further explore the effect of lycopene, beta-carotene and tomato oleoresin on the susceptibility of LDL to oxidation, we used LDL which is highly susceptible to oxidation, obtained from the atherosclerotic, apolipoprotein E deficient (E degrees) transgenic mice. Dietary supplementation of E degrees mice for 6 weeks with 50 mu g of pure lycopene or beta-carotene/mouse/week, or with 1 mg of tomato oleoresin/mouse/week, resulted in a substantial enrichment of their LDL with carotenoids, by 7.5, 3.2 or 2.5 fold respectively. After tomato oleoresin or lycopene supplementation, LDL from the E degrees mice was more resistant to CuSO4-induced oxidation, by 90% and 22% respectively, whereas beta-carotene had only minimal effect on LDL oxidizability. Conclusion: Our results indicate for the first time a protective effect of tomato lycopene against oxidative modification of LDL. This LDL protection by lycopene exceeded the protection exhibited by beta-carotene. However, this effect was selective only to LDLs with high vitamin E content and was potentiated when the carotenoids were present in combination with vitamin E. (C) 1997, Medikal Press.
