Homocysteine strongly enhances metal-catalyzed LDL oxidation in the presence of cystine and cysteine.

Here we show that homocysteine stimulates low density lipoprotein (LDL) oxidation at copper(II) concentrations causing only a slight oxidation of LDL lipids. LDL oxidation by homocysteine and copper(II) is further enhanced in the presence of cystine, although cystine alone does not stimulate LDL oxidation with copper(II). Similarly, a combination of cysteine with homocysteine provoked a more than additive increase of oxidation. Simultaneous presence of cysteine and homocystine also resulted in a more than additive oxidative effect which was not statistically significant, however. Stimulation of LDL oxidation in the presence of homocysteine by cystine was also observed with iron(III) at acidic pH and when LDL oxidation was initiated by azo-compound generated peroxyl radicals. At pH 7.4 histidine is able to prevent LDL oxidation by copper(II) in a thiol mixture similar to the one found in human plasma if present in tenfold excess over homocysteine, but loses its inhibitory effect at higher homocysteine concentrations. The synergistic effect on metal-catalyzed LDL oxidation observed with mixtures of homocysteine and cystine or cysteine sustains the hypothesis that the epidemiological association between raised homocysteine levels and risk of cardiovascular disease is caused by an increase in oxidative stress.