Ionic strength dependence of the inhibition of acetylcholinesterase activity by Al3+

Inhibition of acetylcholinesterase activity by Al3+ has been examined by initial velocity kinetics and by a first-order kinetic method. Both methods yield an inhibition constant of approx. 1.7 mM at 0.1 M ionic strength. The initial velocity study indicates a noncompetitive mechanism of inhibition by Al3+. Inhibition at 10 mM ionic strength shows a Ki of 0.03 mM. Evaluation of the ionic strength dependence concurs with the results of Nolte et al. (Biochemistry 19 (1980) 3705). An effective charge in the binding site of -9 predicts the ratio of inhibition constants at high and low ionic strength. Extrapolation to zero ionic strength gives a Ki0 = 0.34 microM.