GSH determinations by monochlorobimane in γ-GCS_H knockout (KO) and stably transfected wild-type (wt-GCS_H) and mutant (Arg127Cys) cells. As shown, on GSH removal from culture media, γ-GCS_H KO cells (□) quickly deplete in their GSH level. Wild-type γ-GCS_H transfectants (▦) maintain an adequate level of GSH production over the 4-day period, whereas mutant Arg127Cys transfectants (▪) are intermediate in GSH production, approximately 10% to 30% of wild type. Values expressed are a ratio of GSH concentration to cell number and represent 3 measurements. Error bars represent SEM.