Tissue , glucose-6-phosphatase

The liver utilizes glucose-6-phosphatase to remove the phosphate from glucose 6-phosphate. Non-phosphorylated glucose from liver glycogen can traverse the plasma membrane and be transported via the blood circulation to other tissues. Glucose-6-phosphatase is absent from skeletal muscle and brain and so glucose is retained by these tissues as glucose 6-phosphate which cannot permeate the plasma membrane. 

In some tissues, the phosphatase is regulated by hormones. In liver, epinephrine binds to the a-adrenergic receptor to initiate the phosphatidyl inositol pathway (p. 388), causing an increase in Ca" concentration that activates the phosphatase. In tissues capable of fatty acid synthesis, such as the liver and adipose tissue, insulin, the hormone that signifies the fed state, stimulates the phosphatase, increasing the conversion of pyruvate into acetyl Co A. Acetyl CoA is the precursor for fatty acid synthesis (p. 635). In these tissues, the pyruvate dehydrogenase complex is activated to funnel glucose to pyruvate and then to acetyl CoA and ultimately to fatty acids. 

The free glucose produced by this reaction is supplied to the blood from the tissues. As exemplified by gluconeogenesis, one may easily envision the economical organization of these metabolic routes, since, apart from four special gluconeogenesis enzymes-pyruvate carboxylase, phosphopyruvate carboxylase, fructose bisphosphatase, and glucose 6-phosphatase-individual glycolytic enzymes are also used in the gluconeogenesis. 

Glucose-6-phosphatase is found only in liver and kidney, and allows these tissues to supply glucose to other organs of the body, like the brain, which have little or no reserves of carbohydrates. 

It had been known from at least the time of Pasteur that the presence of sodium or potassium phosphate aided the progress of a yeast fermentation. Later intensive study showed that a complex group of enzymes (phosphatases and phosphorylases) was responsible for the phosphorylation, dephosphorylation and interconversion of D-glucose 6-phosphate, D-fructose 6-phosphate, D-fructose 1,6-diphosphate and similar substances in various types of cells and muscle tissue. Detailed reviews of the field are available. - A further advance was made in 1936, when Cori and Cori noted that in certain circumstances well-washed frog muscle immersed in a sodium phosphate buffer utilized the inorganic phosphate to produce a new hexose phosphate (the Cori ester). This compound was later shown to be a-D-glucopyranose-l-phosphate and yielded crystalline dipotassium and brucine salts. The Cori ester arose because... 

Glucose 6-phosphate can then be metabolized by glycolysis in the liver or muscle, or it can be dephosphorylated by the action of glucose 6-phosphatase mainly in the liver and released into the bloodstream for use by other tissues of the body. 

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