Glucose-alanine cycle

Major amino acids emanating from muscle are alanine (destined mainly for gluconeogenesis in liver and forming part of the glucose-alanine cycle) and glutamine (destined mainly for the gut and kidneys). 

Glycogenolysis and glycogen synthesis P-oxidation of fatty acids transamination and deamination of amino acids Cori cycle and glucose-alanine cycle, which recycles substrates between muscle and liver. 

Muscle protein catabolism generates amino acids some of which may be oxidized within the muscle. Alanine released from muscle protein or which has been synthesized from pyruvate via transamination, passes into the blood stream and is delivered to the liver. Transamination in the liver converts alanine back into pyruvate which is in turn used to synthesise glucose the glucose is exported to tissues via the blood. This is the glucose-alanine cycle (Figure 7.11). In effect, muscle protein is sacrificed in order to maintain blood adequate glucose concentrations to sustain metabolism of red cells and the central nervous system. 

FIGURE 18-9 Glucose-alanine cycle. Alanine serves as a carrier of ammonia and of the carbon skeleton of pyruvate from skeletal muscle to liver. The ammonia is excreted and the pyruvate is used to produce glucose, which is returned to the muscle. 

The pyruvate produced by deamination of alanine in the liver is converted to glucose, which is transported back to muscle as part of the glucose-alanine cycle. 

The glucose-alanine cycle. Active muscle functions anaerobically and synthesizes alanine by a transamination reaction between glutamate and pyruvate, The alanine is transported to the liver, where the... 

Grubinko, V.V. and Arsan, O.M. (1995). The role of glucose alanine cycle in the adaptation to ammonia in fish (In Russian). Doklady Natsionalnoy Akademii Nauk Ukrainy 1995 (1), 107-109. 

Alternatively, in skeletal muscle, pyruvate can be transaminated to alanine (which affords a route for nitrogen transport from muscle to liver) in the liver alanine is used to regenerate pyruvate, which can then be diverted into gluconeogenesis. This process is referred to as the glucose-alanine cycle. 

The glucose-alanine cycle between the liver and the musculature is particularly significant. In muscle tissue, ammonia is generated during the degradation of amino acids (particularly the branched-chain amino acids). The transfer of ammonia to pyruvate yields alanine, which is then transported through the bloodstream to... 

Alanine is also produced by the glucose-alanine cycle. 

The alanine released by muscle is also produced by the glucose-alanine cycle, which involves the transport of glucose from the liver to muscle and the return of carbon atoms to the liver as alanine. 

Figure 23.16 PATHWAY INTEGRATION The glucose—alanine cycle. During prolonged exercise and fasting, muscle uses branched-chain amino adds as fuel. The nitrogen removed is transferred (through glutamate) to alanine, which is released into the bloodstream. In the liver, alanine is taken up and converted into pyruvate for the subsequent synthesis of glucose.

Alanine is formed from pyruvate in muscle. After it is transported to the liver, alanine is reconverted to pyruvate by alanine transaminase. Eventually pyruvate is used in the synthesis of new glucose. Because muscle cannot synthesize urea from amino nitrogen, the glucose-alanine cycle is used to transfer amino nitrogen to the liver. 

Cori cycle, 252 decarboxylation, 245 electron transport system, 243 fermentation, 243 gluconeogenesis, 235 glucose-alanine cycle, 254... 

Describe how the glucose-alanine cycle acts to transport ammonia to the liver. 

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