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Sulfhydryl groups during the s phase: Comparison of cells from G1, plateau‐phase G1, and G0

The non‐protein sulfhydryl (NPSH) content of cells moving into S from G1, plateau phase G1, and G0 was measured. Chinese hamster ovary (CHO) cells accumulated in G1 by growth into plateau phase contain only one‐fourth the NPSH concentration of cycling C1 cells or G1 cells accumulated by brief growth in isoleucine‐deficient medium. Upon dilution of plateau cultures with fresh medium, cellular NPSH content increases rapidly, reaching the same level as that in cycling cells within four hours. This increase is prevented by cycloheximide but not by actinomycin D or hydroxyurea. Neither CHO cells cycling in vitro nor salivary gland G0 cells stimulated with isoproterenol in vivo show significant changes in intracellular NPSH concentrations during S phase. This suggests that the concentration of intracellular NPSH (glutathione) remains constant during the cell cycle except when cells are grown to plateau phase in exhausted or deficient medium, in which case normal degradation exceeds synthesis and the gross level falls until fresh medium is provided and synthesis, apparently on preexisting RNA templates, accelerates.