Estimation of whole body protein synthesis in rats by single injection of L-[1-14C]leucine or DL-[1-14C]lysine.

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 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.