Estimation of Whole Body Protein Synthesis in Rats by Single Injection of L-[1-14C]Leucine or DL-[1-14C]Lysine

Abstract

We evaluated a method of estimating whole body protein synthesis in rats. Growing rats were given single injections of L-[1-¹⁴C]leucine, DL-[1-¹⁴C]lysine or d-[1-¹⁴C]lysine via a tail vein. Total ¹⁴C in expired air, body protein, body lipids, perchloric acid soluble material and excreta was recovered after 3 hours. After injections of L-[1-¹⁴C]leucine, 89% of injected ¹⁴C was recovered either in expired air or as protein-bound [¹⁴C]leucine. After correction for radiochemical purity of injected L-[1-¹⁴C]leucine, the total recovery of these two fractions was 95%. Only insignificant amounts of ¹⁴C were recovered in expired air and protein-bound [¹⁴C]lysine after injections of d-[1-¹⁴C]lysine. After correcting for radiochemical purity and stereoisomerism of injected DL-[1-¹⁴C]lysine, we found 70% of the ¹⁴C from L-[1-¹⁴C]lysine in expired air or as protein-bound [¹⁴C]lysine. The results demonstrated that single injections of L-[1-¹⁴C]leucine provided valid estimates of whole body protein synthesis, whereas it was likely that a longer period of ¹⁴CO₂ collection was required after injections of DL-[1-¹⁴C]lysine. By measuring nitrogen balance and absorption of dietary leucine, we estimated the rates of whole body protein synthesis, accretion and degradation to be 3.51, 0.90 and 2.61 g/day, respectively, for those rats injected with L-[1-¹⁴C]leucine.