Development and characterization of hydrogen peroxide-resistant Chinese hamster ovary (CHO) cell variants--II. Relationships between non-protein sulfhydryl levels and the induction/stability of the oxidant-resistant phenotype.

Hydrogen peroxide sensitive and resistant sublines of Chinese hamster ovary (CHO) cells were tested for their non-protein sulfhydryl (NPSH) content in an attempt to establish whether a relationship exists between resistance to growth inhibition elicited by the oxidant and the NPSH pool. Cell variants characterized by increasing levels of resistance to hydrogen peroxide displayed a significant increase in cellular NPSH (expressed on a per million cell basis). Growth of resistant cells for various lengths of time in the absence of H2O2 decreased resistance, whereas the NPSH content did not vary (at least up to 127 days of growth in peroxide-free medium). The NPSH pool returned to control levels after an additional 82 days. These changes, however, were probably related to differences in cell size/amount of total cell proteins in the sublines. Indeed, when NPSH levels were expressed on a per milligram protein basis, essentially no variations were observed in sensitive and resistant sublines. It is important to note that, even by expressing the NPSH content on a per million cell basis, no correlation was found with the degree of resistance to growth inhibition elicited by the oxidant. Further experiments have demonstrated that, under conditions of reduced NPSH content (obtained by growing the cells in the presence of a glutamylcysteine synthetase inhibitor), the cytotoxic action of hydrogen peroxide was very slightly, if at all, augmented in both wild type and resistant cells. We may therefore conclude that cellular NPSH do not afford significant protection against growth inhibition induced by hydrogen peroxide in wild type cells, and that the same lack of effect occurs in cells with an increased NPSH content and carrying the oxidant-resistant phenotype.