Metabolism
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Alanine released from muscle in fasting
Key points from this exercise:
Although most of the body's glycogen is in muscle, this cannot provide a direct source of blood glucose in the fasting state because muscle lacks glucose 6-phosphatase. (In an adult the liver contains ~ 90 g of glycogen in the fed state is and the muscle ~245 g).
In the fasting state muscle puts out amino acids that have arisen form catabolism of muscle protein. Alanine is about 60% of total amino acid output, yet it accounts for only 10% of the total amino acids in muscle proteins.
In the fasting state liver takes up alanine from the bloodstream and uses it as a substrate for gluconeogenesis.
Alanine undergoes transamination to yield pyruvate; the amino group is transferred onto ketoglutarate, forming glutamate, which then undergoes oxidative deamination to reform ketoglutarate and release ammonium ions, which are used for synthesis of urea for excretion.
Muscle glycogen yields glucose 1-phosphate, which undergoes glycolysis to pyruvate. Pyruvate then undergoes transamination to yield alanine, which is exported to the liver. The amino groups come from the amino acids released by catabolism of muscle proteins. The carbon skeletons of the amino acids (the keto-acids) are metabolised as metabolic fuel in muscle.