Relative glutaminase activity in the context of leukemic cell kill. In these cell-kill assays, the asparaginase activity was kept constant. For N24A, N24T, N24A R195S, and N24A Y250L, the relative glutaminase activity (x-axis) weighted by the asparaginase activity and relative IC50 (y-axis) values are shown separately. The cytotoxic activity of each mutant L-ASN is indicated for the leukemic cell lines SupB15 (very drug sensitive, ▩), MV 4:11 (intermediate drug sensitivity, ■), and REH (resistant, □). Values > 1 (dashed line) indicate IC50 values lower than WT compound, values equal to 1 indicate cytotoxicity equivalent to WT L-ASN, and values < 1 indicate a higher IC50 than WT. For each mutant, an asterisk above the identifier indicates significant differences in glutaminase activity. Significant relative differences in cytotoxicity of mutant L-ASN (compared with WT) are indicated by an asterisk on the right side of the data point. *P < .05; **P < .01.