Cytosolic fructose 1,6-bisphosphatase

Chloroplast and cytosolic Fructose-1,6-bisphosphatase (FbPase) and Sedoheptulose-l,7-bisphosphatase (SBPase) have been investigated mainly from spinach leaves (1,2,3). We present data which show significant differences for these enzymes from pea leaves. 

Phosphoenolpyruvate carboxykinase (PEPCK) deficiency is distinctly rare and even more devastating clinically than deficiencies of glucose-6-phosphatase or fructose-1,6-bisphosphatase. PEPCK activity is almost equally distributed between a cytosolic form and a mitochondrial form. These two forms have similar molecular weights but differ by their kinetic and immunochemical properties. The cytosolic activity is responsive to fasting and various hormonal stimuli. Hypoglycemia is severe and intractable in the absence of PEPCK [12]. A young child with cytosolic PEPCK deficiency had severe cerebral atrophy, optic atrophy and fatty infiltration of liver and kidney. 

Phosphoenolpyruvate is converted to fructose-1,6-bisphosphate by reversal of glycolysis in the cytosol via reactions that are at near-equilibrium and whose direction is dictated by substrate concentration. Conversion of fructose-1,6-bisphosphate to fructose-6-phosphate is a nonequilibrium step, catalyzed by fructose-1,6-bisphosphatase ... 

We have isolated and sequenced cDNAs for wheat phosphoribulokinase (PRK), for chloroplast fructose-1, 6-bisphosphatase (FBP) and for both the chloroplast and cj osolic versions of phosphoglycerate kinase (PGK). We are at present sequencing a putative cDNA clone for sedoheptulose-l,7-bisphosphatase (SBP) and isolating the coding sequence of cytosolic FBP. For the sake of comparison, we have also sequenced the E coU gene for FBP (3). 

Fructose 1,6-bisphosphatase (F 1,6-bisPase) in the cytosol is inhibited by fructose 2,6-bisphosphate. Fructose 2,6-bisphosphate is destroyed under the influence of glucagon. Conversely, it is synthesised under the influence of insuhn. 

Investigations of liver biopsy material for enzymes involved in lactic acidosis and ketogenesis showed normal activites of pyruvate carboxylase, citrate synthetase, isocitrate dehydrogenase, glutamate-pyruvate transaminase, reduced activity of fructose 1,6-bisphosphatase and notably reduced activity of cytosolic acetoacetyl-CoA thiolase. The latter was found to be due to altered kinetic properties of the enzyme and this was confirmed in cultured skin fibroblasts, the enzyme being much more sensitive to coenzyme A inhibition than the normal enzyme. Activity of succinyl-CoA 3-keto acid-CoA transferase was not reported. 

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