Abstract
Cytosolic fructose-1,6-bisphosphatase from spinach (Spinacia oleracea L.) leaves was purified over 1700-fold. The final preparation was specific for fructose-1,6-bisphosphate in the presence of either Mg(2+) or Mn(2+), and was free of interfering enzyme activities. Ca(2+) was an effector of fructose-1,6-bisphosphatase activity, and showed different kinetics, depending on whether Mg(2+) or Mn(2+) was used as cofactor. In the presence of 5 millimolar Mg(2+), Ca(2+) appeared as activator or as inhibitor of the enzyme at low or high levels of substrate, respectively. In both cases, a rise in affinity for fructose-1,6-bisphosphate was observed. A model is proposed to describe the complex interaction of fructose-1,6-bisphosphatase with its substrate and Ca(2+). However, with Mn(2+) (60 micromolar) as cofactor, Ca(2+) exhibited the Michaelis-Menten kinetics of a noncompetitive inhibitor. When assayed at constant substrate concentration, Ca(2+) behaves as a competitive or noncompetitive inhibitor, depending on the use of Mg(2+) or Mn(2+) as cofactor, respectively, with a positive cooperativity in both cases. Fructose-2,6-bisphosphate showed a classic competitive allosteric inhibition in the presence of Mg(2+) as cofactor, but this effect was low with Mn(2+). From these results we suggest that Ca(2+) plays a role in the in vivo regulation of cytosolic fructose-1,6-bisphosphatase.