Brain glucose bisphosphatase requires inosine monophosphate

Glucose bisphosphate phosphatase has been partially purified from the cytosol of mouse brain. Enzyme activity required Mg2+ and a heat-stable cofactor. The activator was present in boiled extracts of mouse brain mitochondrial-nuclear fraction, of red blood cells, or of rabbit muscle. The chemical properties of the activator are consistent with its identification as inosine monophosphate (IMP), including its mobility in a high pressure liquid chromatography (HPLC) system capable of resolving all of the biologically important mononucleotides. A large number of other biologically important compounds were not effective, including AMP, cAMP, cGMP, and UMP, GMP, purified by HPLC, (50 or 74 microM), gave a rate about 35% of that obtained with IMP (5 microM). The enzyme was separated completely from phosphoglucomutase and significantly from glucose bisphosphate synthase. The products of the reaction are glucose-P and Pi. Fructose bisphosphate at 500 microM inhibited only 40% in the presence of 20 microM glucose bisphosphate. The activation by IMP follows hyperbolic kinetics with an apparent Ka of 5 microM in the presence of 12 microM glucose bisphosphate. The apparent Km of glucose bisphosphate was 10 microM in the presence of 50 microM IMP. There was no inhibition by 5 or 50 microM AMP or ADP. The possible regulatory importance of glucose bisphosphate in carbohydrate metabolism and the significance of the regulation of the phosphatase by the nucleotide are discussed.