Effect of Rate of Catabolism of DL-[1-14C]Lysine on Estimation of Whole Body Protein Synthesis in Rats

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Abstract
We investigated the effect of rate of DL-[1-14C]lysine metabolism on the estimation of whole body protein synthesis in rats. Growing rats were given single injections of DL-[1-14C]lysine via a tail vein and cumulative expiration of 14CO2 was measured after 3, 6 and 9 hours. Total 14C in protein-bound lysine, body lipids, perchloric acid-soluble material, urine, feces and gastrointestinal contents was recovered after 9 hours. After correcting for radiochemical purity and stereoisomerism of DL-[1-14C]lysine, 95.1% of injected L-[1-14C]lysine was found in expired air and protein-bound [14C]lysine. Daily nitrogen retention of rats was 150 mg and daily incorporation of lysine into body protein was 119% of that absorbed from the gastrointestinal tract, from which rates of whole body protein synthesis, accretion and degradation were estimated to be 2.58, 0.76 and 1.83 g/day, respectively. Expressed per unit of body weight the rate of protein synthesis was not significantly different from previous estimates using L-[1-14C]leucine, which indicated that differences in catabolic pathways of leucine and lysine as well as differences in intracellular specific activities of these [14C]amino acids had a negligible effect on the estimation of protein synthesis.