CELLULAR OXIDATION OF LOW-DENSITY-LIPOPROTEIN IS CAUSED BY THIOL PRODUCTION IN MEDIA CONTAINING TRANSITION-METAL IONS

The oxidation of low density lipoprotein (LDL) may be important in ath erosclerosis. LDL can be oxidized by cultured cells, including macroph ages and endothelial cells. This cellular oxidation is dependent on tr ansition metal ions in the medium. We now report that LDL oxidation by endothelial cells and macrophages is caused by cell-dependent appeara nce of thiol in the medium (''thiol production''). Thiol appeared in m edium when cells were incubated under standard serum-free culture cond itions. L-Cystine in the medium was required for thiol production and also for LDL oxidation. Cell-dependent appearance of thiol was inhibit ed by glutamate (which blocks cystine uptake) and by diethylmaleate (w hich reacts with thiols). Both compounds also blocked cellular LDL oxi dation, even though neither compound had antioxidant activity. Finally , we designed an enzymatic system, based on glutathione reductase, tha t mimicked cellular thiol production. This enzymatic system caused LDL oxidation, and showed the same dependency for transition metal ions a s did cellular LDL oxidation. We conclude that in media containing tra nsition metal ions, cellular oxidation of LDL can be explained by the cell-dependent appearance of thiol in the medium. A very similar mecha nism was proposed in 1987 by Heinecke et al. U Biol. Chem. 262: 10098- 10103). Under other conditions, however, cellular oxidation of LDL may occur by other mechanisms.