Activation of rabbit kidney fructose diphosphatase by Mg-EDTA, Mn-EDTA and Co-EDTA complexes

Abstract Purified rabbit kidney fructose diphosphatase requires both a free cation and a metal-chelate when assayed at pH 8 or below. In the presence Mg 2+ or Mn 2+ , effective metal chelates were Mn(II)-EDTA, Mg(II)-EDTA, and Co(III)-EDTA. With Mg 2+ as the cation the affinity of the enzyme for Mn(II)-EDTA or Mg(II)-EDTA was approximately the same, and 300-fold greater than that for Co(III)-EDTA. Activation of the enzyme by the very stable Co(III)-EDTA complex, as well as failure of an ionophore antibiotic to replace EDTA as activator, exclude the possibility that the effects of EDTA are due to removal of metal inhibitors. Inhibition of fructose diphosphatase by Ca 2+ was competitive with Mg 2+ , and noncompetitive with Mg(II)-EDTA, or Co(III)-EDTA. Conversely inhibition by Zn(II)-EDTA was competitive with Mg(II)-EDTA and noncompetitive with free Mg 2+ . The data suggest that the free metals bind to one site on the enzyme while the metal-EDTA chelates bind to a second site.