The Liver Is Important in Carbohydrate Metabolism

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The liver is extremely important in maintaining an adequate supply of nutrients for cell metabolism and regulating blood glucose concentration (Fig. 28.3). After the ingestion of a meal, the blood glucose increases to a concentration of 120 to 150 mg/dL, usually in 1 to 2 hours. Glucose is taken up by hepatocytes by a facilitated carrier-mediated process and is converted to glucose 6-phosphate and then UDP-glucose. UDP-glucose can be used for glycogen synthesis, or glycogenesis. It is generally believed that blood glucose is the major precursor of glycogen. However, recent evidence seems to indicate that the lactate in blood (from the peripheral metabolism of glucose) is also a major precursor of glycogen. Amino acids (e.g., alanine) can supply pyru-vate to synthesize glycogen.

Glycogen is the main carbohydrate store in the liver, and may amount to as much as 7 to 10% of the weight of a normal, healthy liver. The glycogen molecule resembles a tree with many branches (see Fig. 27.19). Glucose units are linked via a-1,4- (to form a straight chain) or a-1,6 (to form a branched chain) glycosidic bonds. The advantage of such a configuration is that the glycogen chain can be broken down at multiple sites, making the release of glucose much more efficient than would be the case with a straight-chain polymer.

During fasting, glycogen is broken down by glyco-genolysis. The enzyme glycogen phosphorylase catalyzes the cleavage of glycogen into glucose 1-phosphate. Glycogen phosphorylase acts only on the a-1,4-glycosidic bond,


Glucose (present in high

Glucose (present in high

Carbohydrate Metabolism Liver
The regulation of carbohydrate metabolism in the liver.

and the enzyme a-1,6-glucosidase is used to break the a-1,6-glycosidic bonds.

Glucose 1-phosphate is converted to glucose 6-phos-phate by the enzyme phosphoglucomutase. The enzyme glucose-6-phosphatase, which is present in the liver but not in muscle or brain, converts glucose 6-phosphate to glucose. This last reaction enables the liver to release glucose into the circulation. Glucose 6-phosphate is an important intermediate in carbohydrate metabolism because it can be channeled either to provide blood glucose or for glycogen formation.

Both glycogenolysis and glycogenesis are hormonally regulated. The pancreas secretes insulin into the portal blood. Therefore, the liver is the first organ to respond to changes in plasma insulin levels, to which it is extremely sensitive. For instance, a doubling of portal insulin concentration completely shuts down hepatic glucose production. About half the insulin in portal blood is removed in its first pass through the liver. Insulin tends to lower blood glucose by stimulating glycogenesis and suppressing glycogenolysis and gluconeogenesis. Glucagon, in contrast, stimulates glycogenolysis and gluconeogenesis, raising blood sugar levels. Epinephrine stimulates glycogenolysis.

The liver regulates the blood glucose concentrations within a narrow limit, 70 to 100 mg/dL. Although one might expect patients with liver disease to have difficulty regulating blood glucose, this is usually not the case because of the relatively large reserve of hepatic function. However, those with chronic liver disease occasionally have reduced glycogen synthesis and reduced gluconeoge-nesis. Some patients with advanced liver disease develop portal hypertension, which induces the formation of por-tosystemic shunting, resulting in elevated arterial blood levels of insulin and glucagon.

acids, and lactate. The process is energy-dependent, and the starting substrate is pyruvate. The energy required seems to be derived predominantly from the ^-oxidation of fatty acids. Pyruvate can be derived from lactate and the metabolism of glucogenic amino acids—those that can contribute to the formation of glucose. The two major organs involved in the production of glucose from noncarbo-hydrate sources are the liver and the kidneys. However, because of its size, the liver plays a far more important role than the kidney in the production of sugar from noncarbo-hydrate sources.

Gluconeogenesis is important in maintaining blood glucose concentrations especially during fasting. The red blood cells and renal medulla are totally dependent on blood glucose for energy, and glucose is the preferred substrate for the brain. Most amino acids can contribute to the carbon atoms of the glucose molecule, and alanine from muscle is the most important. The rate-limiting factor in gluconeogenesis is not the liver enzymes but the availability of substrates. Gluconeogenesis is stimulated by epi-nephrine and glucagon but greatly suppressed by insulin. Thus, in type 1 diabetics, gluconeogenesis is greatly stimulated, contributing to the hyperglycemia observed in these patients (see Chapter 35).

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  • Tanja
    How liver regulates blood glucose concentration?
    8 years ago
  • haben demsas
    Why is liver important in carbohydrate metabolism?
    4 years ago
  • stella
    Why is liver important in carbohydrates metabolism?
    3 years ago
  • jennifer
    Why is liver imporatant in carbohydrate metabolism?
    3 years ago
  • aira
    Why the liver is important in carbohydrate metabilism?
    3 years ago
  • Mustafa Daniel
    Why liver is important in carbohydrats metabolism?
    3 years ago
  • rebekah
    Why liver is importantin carbohydrates metabolism?
    3 years ago
  • Sonja
    Why lever is important in carbohydrate metabolism?
    3 years ago
  • Yorda
    Why liver is important in carbonhydrate metabolism?
    3 years ago
  • Sanna Yli-Hannuksela
    Why is the liver importantin carbohydrate metabolism?
    3 years ago
    What are the two process used by the liver in carbohydrates metabolism?
    3 years ago
    What is the function of liver on carbohydrates metabolism?
    3 years ago
  • benedetta
    Why is my liver metabolism enzymes?
    3 years ago
  • hanna
    What Are The Important Organ For Carbohydrate?
    3 years ago
  • andrew black
    Why all the carbohydrates metabolism occur in the liver?
    3 years ago
  • Matthias
    Which two organs are important in glucose metabolism?
    3 years ago
  • bryan
    Why is liver important organ of glucose?
    2 years ago
  • Timo
    Why is liver important for glucose metabolismmeta?
    2 years ago
  • filomena
    Why liver is an important organ for glucose metabolism?
    2 years ago
  • belisarius oldbuck
    How does the liver assist in carbohydrate metabolism?
    2 years ago
  • Hannes
    What is the importance of liver in carbohydrate metabolism?
    2 years ago
  • Barbara
    Why is eating liver important?
    2 years ago
  • Jodie
    How liver disease can affect carbohydrate metabolism?
    2 years ago
  • robin
    What are the chief carbohydrates of liver?
    2 years ago
  • lalia
    Why liver is important in carbohydrate matabilism?
    2 years ago
  • jessamine greenhand
    Why the liver is important for carbphydrates metabolism?
    2 years ago
  • Elda
    2 years ago
  • gilly
    Why is the liver importasnt in metobalism?
    2 years ago
  • Tomba
    Why is glucose6phosphate important in metabolism?
    2 years ago
  • tamara
    What is the role of hepatocytes in carbohydrates metabolism?
    1 year ago
  • jackie
    1 year ago
  • amy poindexter
    Why are levers so important?
    1 year ago
  • Elanor
    Why does liver glucose decrease when carbohydrate in take increases?
    7 months ago
  • Christian
    Why liver is vital organ with respect to glucose metabolism?
    7 months ago
  • Oskari
    Why is the liver important in glycolysis?
    5 months ago
  • Dudo
    What is the importance of carbohydate metabolism?
    5 months ago
  • matilda
    Why is the liver important incarbohydrates metabolism?
    3 months ago
  • tuulia
    Why liver is important in carborn metabolism?
    3 months ago
    Why lover is important in carbohydrate metabolism?
    2 months ago
  • anselmo
    Why the liver is important in carbohydrate meterbolisim?
    2 months ago
  • carla
    When your muscles and liver are full of company hydrates an extra carbohydrates or converted into?
    2 months ago
  • robert
    How is the liver involved in carbohydrate metabolism?
    26 days ago
  • kalvin
    Which metabolism in liver requires glucose?
    25 days ago
  • cara
    Why is the liver important in vorbohydrate metabolism?
    23 days ago
    Why the liver is important i carbydrate metaolism?
    15 days ago

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