Catabolism and synthesis of amino acids in skeletal muscle: their significance in monogastric mammals and ruminants [review]
1993
Teleni, E. (James Cook Univ., Townsville (Australia). Dept. of Biomedical and Tropical Veterinary Sciences)
The glucose-alanine cycle, as it was originally proposed, has been well substantiated by studies using human and rat muscles. An alternative proposal, which suggested that other amino acids make a major contribution to the carbon skeleton of alanine synthesized in muscle, is less convincing. In the ruminant, the glucose-alanine cycle is quantitatively less significant than in the human. The probable reason for this difference is the limited available pyruvate in ruminant muscle for transamination to alanine. This may be due to a lower carbon flux through the glycolytic pathway and-or to significant activity of the anaplerotic enzyme, pyruvate carboxylase. It is suggested that glutamine is the more important carrier of carbon and nitrogen out of skeletal muscle and that alanine may serve only as an ancillary vehicle to transport carbon and nitrogen when the availability of pyruvate for transamination in muscle is high. The more diverse role of glutamine (cf. alanine) in acid-base balance, as a respiratory fuel in cells of the immune system and the epithelia of the small intestine, where it may also be converted to alanine for subsequent gluconeogenesis in the liver, is consistent with this suggestion.
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