Early A-V difference studies across the human leg or arm revealed that more than 50% of the amino acids released from skeletal muscle are in the form of alanine and glutamine (132), yet alanine and glutamine comprise less than 20% of amino acids in protein (T§ble..2...,3). The specific purpose of the alanine and glutamine release is still not well defined. Several possible reasons exist. First, skeletal muscle will oxidize nonessential amino acids and the BCAAs in situ for energy. Because amino acid oxidation produces waste N and because ammonia is neurotoxic, release of waste N as ammonia must be avoided. Because both alanine and glutamine are readily synthesized from intermediates derived from glucose (alanine from transamination of pyruvate from glycolysis, and glutamine from a-ketoglutarate), they are excellent vehicles to remove waste N from muscle while avoiding ammonia release. Alanine removes one and glutamine removes two Ns per amino acid. These observations have led to the proposal of a glucose-alanine cycle in which glucose made by the liver is taken up by muscle, where glycolysis liberates pyruvate. The pyruvate is then transaminated to alanine and released from muscle. That alanine is extracted by the liver and transaminated to pyruvate that is then used for glucose synthesis (132). This scheme has been expanded to explain the use of BCAAs by muscle for energy and the disposal through alanine of their amino-N groups. Such a scheme resolves a problem related to the BCAAs. In contrast to the other essential amino acids, which are metabolized only in liver, the BCAAs are readily oxidized in other tissues, especially muscle. Thus, if the BCAAs, which comprise about 20% of amino acids in muscle protein, are oxidized for energy by the muscle, then the glucose-alanine cycle could provide a means of removal of the N that is generated in the process.
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