Pathways of carbohydrate metabolism
Uptake of glucose into the cells is the limiting step in its utilization in many tissues, including muscle, heart, and adipose tissue. Insulin is essential for glucose entrance to these tissues, whereas the process is independent of insulin in the liver and CNS.
Equally important and possibly related to glucose uptake is its phosphorylation to glucose-6-phosphate by herokinases before it can enter the metabolic pathways of the cell. This reaction is practically irreversible in most tissues. Once glucose-6-phosphate is formed, it must enter the metabolic pathways and cannot be returned to the blood except from the liver and kidney, where another enzyme, glucose-6-phosphatase, can release free glucose. Glucose-6-phosphate serves as a link behveen the major pathways of glucose metabolism.
In contrast to the extrahepatic tissues, insulin regulates the metabolism of glucose in the liver after its uptake by affecting the activities of hexokinase and other hepatic enzymes.
