In the presence of glucose, yeast hexokinase is specifically and strongly inhibited by all MIIIATP (M = metal) complexes that do not hydrolyze at neutral pH, as long as the ionic radius of the metal is less than 0.89 A. Ki values vary from the micromolar range (0.16 microM for AlATP at pH 7, for example) to as low as 13 nM for LuATP. With glucose and fructose, the tightly bound complexes also show reversible, slow binding behavior, but with poor substrates, little or no change in inhibition constant with time is observed. The kinetics of citrate as an activator of the hexokinase reaction are consistent with its reaction with AlATP present as a contaminant in commercial ATP to form Al citrate. The complex of Al(III) with citrate is 5 orders of magnitude more stable than AlATP, whose Kd is 0.7 microM at pH 7. ATP that has been treated with excess EDTA and adsorbed on and eluted from charcoal is free of aluminum, and citrate no longer affects the kinetics of the hexokinase reaction. Glycerokinase is also specifically inhibited by trivalent metal ATP complexes (Ki = 4 microM at pH 7 for AlATP).