Wild-type, V389L, T103S, and T103I GCKs incubated with 0.5, 1.0, and 1.5 units of GKRP. GCK activity was normalized to a control in which the reaction conditions were identical, but GKRP was absent. IC50 values were calculated using these data and defined as the half-maximal inhibitory concentration of GKRP in each experiment. Data are shown as means ± S.E. for n = 6 experiments and are representative of two preparations of wild-type and each mutant protein.