Fructose 1, 6-bisphosphate aldolase [EC 4.1.2.13] in rat liver was found to be bound to the intracellular membraneous structures such as microsomes and nuclear membranes when the animals were fasted for 48hr 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-phos-phate 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 8×10
-6, 6.1×10
-6, and 4.8×10
-6M, respectively, at pH 7.3. With the microsome-bound enzyme,
Kd values of 3.9×10
-4, 4.1×10
-4, 2.7×10
-3, 1.1×10
-2 and 2.0×10
-2 M were obtained for glyceraldehyde 3-phosphate, dihydroxyacetone phosphate, fructose 1-phosphate, fumarate, and KCl, respectively. Strong cooperativities were observed in the enzyme desorption by the substances which showed large Ka values.
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