Divalent cations and chelators as regulators of brain fructose-1,6-bisphosphatase

Purified fish and rat brain FruP2ase(s) are stimulated by a number of chelators, viz., histidine, EDTA, citrate, imidazole, and a number of histidine analogues. These also impart 5'-AMP sensitivity to the otherwise insensitive enzyme. Beyond 3 mM concentration, histidine inhibits the enzyme activity, which can be prevented by Mn2+. Atomic absorption spectrophotometry showed the presence of 5-6 mol of Mn2+ and Zn2+ bound to both fish and rat brain FruP2ase, which can be removed by exhaustive EDTA-dialysis. The EDTA-dialyzed brain FruP2ase records an absolute Mn2+ requirement and 5'-AMP sensitivity without any chelator treatment. The 5'-AMP sensitivity of such enzyme is abolished by prior incubation with Zn2+. The Zn(2+)-treated brain FruP2ase fails to bind to a Blue-Sepharose column, in contrast to that seen using the untreated enzyme. These results suggest that rat and fish brain FruP2ase(s) are actually Mn(2+)- and Zn(2+)-containing proteins with Zn2+ bound at or near the nucleotide-binding site.