Glycogen metabolism glycogenesis

As work with vanadium compounds and diabetes in cell system has continued, it has become clear that there are also insulin-independent mechanisms at work. One insulin-independent signal transduction pathway appears to be involved in glycogen metabolism reactions in rat adipocytes [137] that also involve PI-3K. A major difference was that only vanadate promoted glycogenesis through the activation of a cytosolic protein tyrosine kinase, which was mediated in an insulin receptor-independent manner. 

Eleven defects in the metabolism of glycogen have been reported nine of them affect skeletal muscle directly (see Figure 5), but only glycogenosis type II (acid maltase deficiency) and glycogenesis type V (myophosphorylase deficiency) are reasonably common the rest are rare and some have been recorded in isolated case studies only. 

Although it is absorbed more slowly than dextrose from the gastrointestinal tract, fructose is metabolized more rapidly. Metabolism of fructose occurs mainly in the liver, where it is converted partially to dextrose and the metabolites lactic acid and pyruvic acid. Entry into the liver and subsequent phosphorylation is insulin-independent. Further metabolism occurs by way of a variety of metabolic pathways. In healthy and well regulated diabetics, glycogenesis (glucose stored as glycogen) predominates. 

The hormone insulin, produced by the pancreas in response to high blood glucose levels, stimulates the synthesis of glycogen, glycogenesis. Insulin is perhaps one of the most influential hormones in the body because it directly alters the metabolism and uptake of glucose in all but a few cells of the body. 

Fructose metabolism in muscle is illustrated in Fig. 22.2. Fructose is phosphorylated by hcxokina.sc to fructose 6-phosphatc. The fructose 6-phosphate is then used for glycogenesis or, when the glycogen reserves are fuU, energy metabolism via glycolysis. 

The possible conversion of fat to carbohydrate within the organism has been a subject of much controversy. While the glycerol component of the fat molecule can be transformed readily into liver glycogen, a carbohydrate precursor has not yet been identified among the products of fatty acid metabolism. The existence of a pituitary factor in carbohydrate metabolism has been established, and one of its effects is ascribed to promotion of glycogenesis from fatty acids. 

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