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

Abstract

Abstract The non‐protein sulfhydryl (NPSH) content of cells moving into S from G 1 , plateau phase G 1 , and G 0 was measured. Chinese hamster ovary (CHO) cells accumulated in G 1 by growth into plateau phase contain only one‐fourth the NPSH concentration of cycling C 1 cells or G 1 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 G 0 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.