Kinetic properties of cytosolic fructose 1,6-bisphosphatase from grapefruit. Effect of citrate.

cFBP is studied for its affinity to Mg++ and Fru-1,6-P2. The affinity for Mg++ is not very high with a Km of 0.24 +/- 0.01 mM. High concentrations of Mg++ are inhibitory. The saturation curve for Fru-1,6-P2 is hyperbolic with a Km of 0.54 +/- 0.014 microM. The presence of citrate (10 mM) induces a sigmoidal curve, modifying both Vmax and S0.5. Citrate affects the allosteric properties of cFBPase: at low substrate concentration cooperativity becomes negative while at higher concentration it is positive. Addition of higher concentrations of Mg++ shows a synergistic effect with citrate, decreasing of the affinity for Fru-1,6-P2: S0.5 equals 7.6 +/- 0.25 mM, 9.0 +/- 0.86 mM and 21.5 +/- 1.46 mM in presence of 5, 7.5 and 10 mM Mg++, respectively.