The metabolic fates of amino acids and the formation of glutamine in skeletal muscle.

These studies have examined whether skeletal muscle during fasting uses amino acids derived from muscle protein breakdown as an energy source or as precursors in the synthesis of glutamine and alanine. Diaphragms from fasting rats were incubated in vitro to study the fates of aspartate, asparagine, glutamate, isoleucine, and valine, the only amino acids that can be converted to tricarboxylic acid cycle intermediates in muscle. Although these five amino acids comprise about 28% of the residues in diaphragm protein, they accounted for only about 14% of the amino acids released into the medium. The fraction (50%) of these amino acids generated by protein breakdown but not released by the tissue may be accounted for by conversion to glutamine. Although glutamine comprises only about 6% of the amino acids in muscle protein, it accounted for about 25% of the amino acids released. This additional amount of glutamine was equivalent to the missing amount of aspartic acid, asparagine, glutamic acid, isoleucine, and valine. Diaphragms were incubated with L-[UJ4C]valine, L-[U-