Expression of phospholipid hydroperoxide glutathione peroxidase in human tumor cell lines

Free radical-induced damage in cells is thought to be involved in the initiation and/or promotion of many cancers. A multilevel antioxidant defense system effective in the protection against free radical damage includes chain-breaking antioxidants and enzymes, such as superoxide dismutase, catalase, and glutathione peroxidases. These latter, glutathione peroxidase (PGX) and phospholipid hydroperoxide glutathione peroxidase (PHGPX), are the only selenoperoxidases present in cells and have been distinct on the basis of mol wt, substrate specificity, and peptide and cDNA fragment sequencing. The ability of PHGPX to reduce phospholipid and cholesterol hydroperoxides accounts for its ability, which is not shared with GPX, to protect membrane from lipid peroxidation, in cooperation with vitamin E. PHGPX was identified in a wide range of mammalian tissues-red blood cells being a notable exceptionand has been recently detected in human tumor-derived cells lines-MCF 7, K 562, HL 60, their doxorubicin resistant variant ADR R, and Hep G2after growth in selenium-sufficient medium. PHGPX specific activity in these cells ranged between 7-22 nmol/min/mg protein. The location of the enzyme was found to be mainly cytosolic, but 3-10% of the total activity was detected in the membrane fraction, whereas 15% of the total activity was detected in the nuclear fraction. The cytosolic enzyme was identified among the radiolabeled selenioproteins present in HEP G2 and