Effect of Ca2+ on MAO-A substrate kinetics in HT-22 hippocampal cell line. (A) HT-22 cell lysates (100 μg/100 μL; n = 3) were incubated with increasing concentrations of substrate, e.g. [14C]-5-HT in the absence (○) or presence (●) of Ca2+ (1 mM). The individual curves represent data pooled from three independent experiments. Data are presented as mean ± SD. (B) The highlighted part of the curves in (A) is expanded to demonstrate the change in Km. (C) Data presented on a double-reciprocal plot clearly demonstrate a change in slope (e.g. Km/Vmax) with no change in y-intercept (e.g. 1/Vmax).