Relationships between amino acid catabolism and protein anabolism in the ruminant.

The observed relation found in sheep between the flux rate of an amino acid and the proportion found in whole-body protein suggests that the major immediate fate of an amino acid is its incorporation into tissue protein. This may be true even for dispensable amino acids. In ruminants, there is substantial utilization of several amino acids (serine, glycine, threonine, histidine, and methionine) for the synthesis of methyl groups; the use of these amino acids for gluconeogenesis is limited. There is little evidence that demands of gluconeogenesis limit the availability of amino acids for protein synthesis. Most amino acids are catabolized in the liver but there may be significant catabolism of alanine, aspartate, and glutamate in peripheral tissues, especially muscle. Normally, peripheral catabolism of branched-chain amino acids is significantly less in ruminants than other species. Nevertheless, there is some oxidation of leucine by muscle and this may be substantially increased in the diabetic state. Catabolism of leucine (and perhaps isoleucine and valine) might be inversely related to use for protein synthesis, but there is no evidence of such a relation for other amino acids.