Kidney fructose-1,6-diphosphatase

Fructose diphosphatase has also been purified from acetone powders of rabbit kidney (63) by acid precipitation, ammonium sulfate precipitation, heating at pH 4.5, and chromatography on phosphocellulose, and sulfoethyl Sephadex. This preparation, which was homogeneous in gel electrophoresis, was employed for the immunological studies described earlier. 

Although gluconeogenesis is generally considered to be confined to liver and kidney, evidence for the presence of a specific FDPase in muscle has been reported from a number of laboratories. Significant levels of activity are to be found in skeletal muscle of a wide variety of vertebrates including mammals, birds, and amphibia (71, 72). The levels of activity in white muscle were reported to be similar to those found in liver and kidney, but the enzyme was not detected in heart muscle or in smooth muscle of several species tested. Fructose diphosphatase in crude muscle extracts has been reported to be stimulated by EDTA (72). 

Fructose 1,6-diphosphatase hydrolyzes D-fructose 1,6-diphosphate to give D-fructose 6-phosphate and PO . It is a key enzyme in the gluconeo-genesis pathway. Two divalent metal ions (Mg2+, Mn2+, Zn2+, and Co2+) are involved in catalysis. In the enzyme isolated from pork kidney the metal-metal distance accounts to 3.7 A [12]. A reaction mechanism similar to that of protein phosphatase 1 was proposed, but leaving group stabilization by metal coordination of the ester oxygen atom appears to be absent (Figure 6) [12]. 

Fructose 1,6-diphosphatase can be isolated from many mammalian systems, including rabbit liver,360 muscle,361,362 rabbit kidney,363 swine... 

D-Fructose 1,6-diphosphatases from rabbit liver and muscle are similar in their cation-requirement profile, molecular weight, substrate affinity, and substrate inhibition, but have different amino acid compositions, and the muscle enzyme does not cross-react with antibody to the purified liver-enzyme.361,364 The muscle enzyme is more sensitive to AMP than the enzyme from liver or kidney.385... 

The binding of AMP, D-fructose 1,6-bisphosphate, and ATP to the kidney 1,6-diphosphatase markedly influences the binding of each of the others. ATP and AMP both increase the affinity of the enzyme for D-fructose 1,6-bisphosphate. In contrast, a decrease in the affinity of the enzyme for ATP is observed when AMP or D-fructose 1,6-bisphosphate is present. ATP decreases the affinity of the enzyme for AMP, and D-fructose 1,6-bisphosphate increases the affinity of the enzyme for AMP. These cooperative interactions between the AMP, ATP, and D-fructose 1,6-bisphosphate binding-sites suggest that the structural alterations that occur in the enzyme upon binding of any one compound leads to changes in the affinity of the enzyme for other compounds. 403,404... 

It was also beheved that the gluconeogenic machinery, including four critical enzymes pyruvate carboxylase, PEP carboxykinase, fructose-1,6-diphosphatase, and glucose-6-phosphatase could be fuUy expressed exclusively in the liver and in the kidney. Specifically, with the exception of hepatocytes and renal cortical cells, it was considered impossible that any glucose synthesized de novo or released Irom local glycogen stores could be released in the systemic circulation, because of the absence of gJucose-6-phosphatase. 

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