Susceptibility of plasma low- and high-density lipoproteins to oxidation in patients with severe hyperhomocysteinemia.

A moderate increase in plasma homocysteine is increasingly considered an important risk factor of atherosclerosis and thrombosis. However, the mechanisms by which hyperhomocysteinemia induces vascular disease are not well defined. In vitro studies suggest that cysteine and homocysteine can induce oxidative modification of low-density lipoproteins (LDL). This suggestion is relevant because lipoprotein oxidation is thought to play a key role in the development of atherosclerosis and in the triggering of thrombotic events. An attractive model to study this topic is provided by patients with classical homocystinuria, an inherited disease characterized by severe hyperhomocysteinemia and a high incidence of thromboembolisms. We investigated the existence of oxidized LDL and the susceptibility to oxidation of the plasma cholesterol-rich lipoproteins in six patients with severe hyperhomocysteinemia, most likely due to classical homocystinuria, and compared the results with matched controls. The proportion of electronegative LDL and the concentration of thiobarbituric acid reactive substances in native LDL and high-density lipoproteins (HDL) did not differ between patients and controls, suggesting that the proportion of modified lipoproteins is not increased in patients with severe hyperhomocysteinemia. The susceptibility to oxidative modification of plasma LDL and HDL was also similar in the two groups, although the patients had homocysteine levels 18.3-fold higher than controls. Thus, increased oxidative modification is not likely to be a relevant mechanism in explaining their high incidence of vascular disease. A possible explanation for the lack of increased susceptibility to oxidation, as would be expected for the metabolic blockade that cause classical homocystinuria, is the 4.1-fold decrease in the concentration of cysteine in the plasma of patients. As a result the total concentration of homocysteine plus cysteine was slightly lower in patients than in controls. This interpretation implies that more studies are needed on lipoprotein susceptibility to oxidation in patients in which both plasma homocysteine and cysteine concentrations are increased. This metabolic situation may be frequent in the population with moderate hyperhomocysteinemia and vascular disease.