Oxidation of amino acids by diaphragms from fed and fasted rats.

GOLDBERG, ALFRED L., AND RICHARD ODESSEY. Oxidation of amino acids by diaphragms from fed and fasted rats. Am. J. Physiol. 223(6) : 1384-1391. 1972.-The oxidation of L-amino-14C acids was studied in isolated diaphragms from fed and fasted rats. During a 90-min incubation, diaphragms from fed rats produced 14C02 from leucinel14C, isoleucine1 -14C, valine1 -14C, alanineP4C, glutamate-U-14C, and aspartate-U-14C at a comparable or greater rate than they incorporated label into protein. By contrast, muscle degraded to CO2 less than 5% of the glycine, serine, and proline entering the tissue and none of the threonine, lysine, methionine, phenylalanine, histidine, tyrosine, and tryptophan. In diaphragms of animals fasted for 3 days, protein synthesis was 50% lower than control values, whereas oxidation of leucine, isoleucine, and valine increased three-to fivefold. Total uptake of these amino acids into the diaphragm also increased. The fall in amino acid incorporation was evident within 1 day after food deprivation, but amino acid oxidation did not increase until the 2nd day. A greater rate of leucine oxidation in such diaphragms occurred over a wide range of leucine concentrations. Upon refeeding the fasted animals, protein synthesis in the diaphragm increased and leucine catabolism decreased to control values within 24 hr. Increased leucine catabolism was also observed in kidney slices from the starved rats but not in liver or brain slices. It is suggested that the increased capacity of kidney and muscle to degrade branched-chain amino acids is an important adaptation to starvation.