The role of thiols in oxidation of low-density lipoprotein by macrophages.

The oxidation o i low-density lipoprotein (LDL) by macrophages. smooth muscle cells antl enclothelial cells. within the vessel wall. may play a central role in the development of atherosclerotic lesions[ I]. Proposed mechanism of cell-mediated oxidation. include attack by lipoxygenases[2] or generatiori of supcroxide[3] but these mechanisms have proved controversial[4.5]. Nitric oxide protects against cellmediated oxidation of LDL[6]. but niay. in the presence of superoxide. fonn pcroxynitrite(7] which can convert LDL to a form recognised by the niacrophage scavenger receptor[ 8). The production of extracellular rhiols. in the presenu: of transition metals. appears to be one mechanism by which niacmphages[9] and smooth muscle cells[ I 0 1 niay oxidize LDL. We have shown that the prntluction of thiols, and the oxidation of LDL by human (THP1) macrophages, is dependent upon the presence of cystine in Hams F10 culture niecliuni. Macrophage-mediated oxidation of LDL was perfomied as described[ 1 I], and measured as TBARS (nmol/nig LDL) antl Relative Electrophoretic Mobility. Production of extracellular thiols. measured as described[9. I I] , was essentially linear during a 24h incuhation. Increasing concentrations of cystine. added to Hams F l 0 lorniulatetl without cystine. increased both thiol production antl LDL oxidation (without affecting cell-free oxidation of LDL). Oxidation of LDL appeared to require a threshold level of thiol production, before plateauing at higher concentrations of extracellular thiol (Figure 1 ).