Modification of low density lipoproteins by erythrocytes and hemoglobin under hypoxic conditions

Oxidation of low density lipoprotein (LDL) has been implicated in atherogenesis. It has also been suggested that modification of LDL in the presence of endothelial and smooth muscle cells is associated with the production of superoxide. Red cells and hemoglobin have been shown to be a source for enhanced superoxide production under hypoxic conditions. We now show that incubation of LDL with both hemoglobin and erythrocytes under hypoxic conditions produces the increased Relative Electrophoretic Mobility (REM) associated with LDL oxidation. With hypoxic hemoglobin, this reaction is over within 10 minutes, appreciably faster than other in vitro methods for LDL oxidation. The increased REM was found to be associated with partial deoxygenation of hemoglobin indicative of appreciable oxygen utilization and a more hypoxic state. At later times, the modified LDL was found to produce enhanced hemoglobin oxidation. The resultant modified LDL was shown to have elevated TBARS indicative of LDL oxidation. In addition, it was found to induce smooth muscle cell proliferation which is one of the biological factors thought to be associated with atherogenesis. The relatively rapid LDL modification detected with hypoxic erythrocytes and hemoglobin suggest that even under in vivo conditions with the antioxidants present in plasma, oxidation may still occur in the circulation with the associated vascular damage occurring as the blood containing elevated levels of oxidized LDL leave the pulmonary circulation.