Age-Related Changes in Whole-Body Amino Acid Kinetics and Protein Turnover in Rats

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Abstract
We studied whole-body amino acid kinetics in rats of approximately 70, 160, 315 and 586 g body weight. The animals were infused simultaneously with L-[U-14C]lysine, L-[1-14C]leucine, L-[U-14C]tyrosine, L-[U-14C]threonine and L-[U-14C]histidine. The plasma flux of each amino acid was estimated from its plasma specific radioactivity at the end of the infusion. Indirect estimates of whole-body protein turnover were calculated, based on the plasma flux of each amino acid. Body protein synthesis was determined from the ratio of the specific radioactivities of the protein-bound and free lysine in tissue homogenate (direct method). Amino acid oxidation was calculated from the difference between the alimentary intake of each amino acid and the amount of each retained in protein. Whatever the amino acid, the plasma flux decreased progressively during growth. Positive correlations were obtained between the plasma flux of the various amino acids. The difference in indirect estimates of wholebody protein turnover obtained with different amino acids was relatively small and did not exceed 40%. At each age, the plasma flux of the various amino acids was related to their content in whole-body protein. The alimentary intake accounted for 25 to 40% of the flux for all amino acids except tyrosine (16 to 28%). The amount of each amino acid that was oxidized generally accounted for 17% of the flux. Rate of protein synthesis based on direct measurement with lysine decreased from 25 to 8.5%/d between 70 and 586 g body weight. Whole-body protein synthesis [g/(d·100 g body weight)] increased by 3.9 for each 1-g increase in daily protein deposition per 100 g body weight.