Metabolism of hydroperoxy-phospholipids in human hepatoma HepG2 cells

Two enzymatic mechanisms have been proposed for the metabolism of hydroperoxy-phospholipids: i) the combined action of phospholipase A2 and glutathione peroxidase, and/or ii) direct enzymatic reduction. The latter reaction may be catalyzed by selenium-dependent phospholipid hydroperoxide glutathione peroxidase and/or by glutathione S-transferase alpha. To study the pathway of this reaction, we used human hepatoma HepG2 cells into which was incorporated labeled, hydroperoxy-phospholipids. The major product of incorporated l-palmitoyl-2-(13-hydroperoxy-cis-9, trans-11-octadecadienoyl)-L-3-phosphatidylcholine was the corresponding hydroxy-phospholipid with no hydroxy- or hydroperoxy-fatty acids. The contributions to reduction of hydroperoxy-phospholipids in HepG2 cells from glutathione S-transferase Al and phospholipid hydroperoxide glutathione peroxidase were calculated to be 0.5% and 99.5%, respectively. Increasing selenium in the cell culture medium led to increases in selenium-dependent phospholipid hydroperoxide glutathione peroxidase activity but not in glutathione S-transferase alpha. This increase in the selenium-dependent enzyme was paralleled by a concomitant increase in the extent of reduction of the incorporated hydroperoxy-phospholipid. We conclude that the main metabolic fate of hydroperoxy-phospholipids in HepG2 cells is by direct reduction to hydroxy-phospholipids by phospholipid hydroperoxide glutathione peroxidase but also by glutathione S-transferase alpha, and that phospholipase A2/selenium-dependent glutathione peroxidase does not play a significant role in the reduction.