Membrane-Bound Fructose 1, 6-Bisphosphate Aldolase: Catalytic Activity and Mechanisms of Desorption

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Abstract
Fructose 1,6-bisphosphate aldolase [EC 4.1.2.13] in rat liver was bound to the intracellular membraneous structures such as microsomes and nuclear membranes when the animals were fasted for 48 h or administered tryptophan. Upon refeeding the rats the aldolase was released into the cytosol. The membrane-bound aldolase was almost inactive, showing about 50-fold larger Km and a smaller Vmax (37%) as compared with those of the free enzyme. The enzyme was released cooperatively from the membrane by exposure to fructose 1,6-bisphosphate, glyceraldehyde 3-phosphate or dihydroxyacetone phosphate at low concentrations. Apparent desorption constants Kd (concentrations necessary for 50% desorption of enzyme) for fructose 1,6-bisphosphate of the enzymes bound to microsomes, mitochondria and nuclei were estimated to be 8 .times. 10-5, 6.1 .times. 10-6 and 4.8 .times. 10-6 M, respectively, at pH 7.3. With the microsome-bound enzyme, Kd values of 3.9 .times. 10-4, 4.1 .times. 10-4, 2.7 .times. 10-3, 1.1 .times. 10-2 and 2.0 .times. 10-2 M were obtained for glyceraldehyde 3-phosphate, dihydroxyacetone phosphate, fructose 1-phosphate, fumarate and KCl, respectively. Strong cooperativities were observed in enzyme desorption by substances which showed large Kd values.

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