tailieunhanh - Basic medical endocrinology - part 5

Do đó các hiệu ứng mạnh mẽ của insulin trên sự tổng hợp glycogen cơ đạt được do tác động bổ sung đường vận chuyển tăng, tăng phosphoryl hóa glucose, và hoạt động enzym tổng hợp glycogen tăng. Số phận thay thế đường-phosphate-6, trao đổi chất để pyruvate trong con đường glycolytic, cũng tăng insulin. | Insulin 187 the balance in favor of dephosphorylation in part by inhibiting the enzyme glycogen synthase kinase 3 GSK-3 and in part by activating a phosphatase. Dephosphorylation of glycogen synthase not only increases its activity directly but also increases its responsiveness to stimulation by its substrate glucose-6-phosphate. Hence the powerful effects of insulin on muscle glycogen synthesis are achieved by the complementary effects of increased glucose transport increased glucose phosphorylation and increased glycogen synthase activity. The alternative fate of glucose-6-phosphate metabolism to pyruvate in the glycolytic pathway is also increased by insulin. Access to the glycolytic pathway is guarded by phosphofructokinase whose activity is precisely regulated by a combination of allosteric effectors including ATP ADP and fructose-2 6-bisphosphate. This complex enzyme behaves differently in intact cells and in the broken cell preparations typically used by biochemists to study enzyme regulation. Because conflicting findings have been obtained under a variety of experimental circumstances no general agreement has been reached on how insulin increases phosphofructokinase activity. In contrast to the liver the isoform of the enzyme that forms fructose-2 6-bisphosphate in muscle is not regulated by cyclic AMP. The effects of insulin are likely to be indirect. It should be noted that oxidation of fat profoundly affects the metabolism of glucose in muscle and that insulin also increases all aspects of glucose metabolism in muscle as an indirect consequence of its action on adipose tissue to decrease FFA insulin concentrations are low increased oxidation of fatty acids decreases oxidation of glucose by inhibiting the decarboxylation of pyruvate and the transport of glucose across the muscle cell membrane. In addition products of fatty acid oxidation appear also to inhibit hexokinase but recent studies have called into question the relevance of earlier

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