[Distribution of radioactivity in the body and rate of incorporation of radioactivity into the tissue proteins of monogastric animals following intravenous injection of tracer amino acids].

The studies were carried out with pigs and rats. The radioactive animo acids (14C leucine and 3H lysine) were administered to the pigs by way of a catheter tube into the jugular vein. Subsequently, the time pattern of the distribution of the specific amino acid radioactivity was followed in the TCE soluble and Tce precipitable fractions of the blood plasma (TCE= trichloro-acetic acid). The radioactive labelling in rats was carried out by injecting 14C leucine into the portal vein. The animals were killed after incorporation periods from 2 to 60 mins, and the levels of specific radioactivity were estimated in the TCE soluble and TCE precipitable fractions of the blood plasma, in the liver and in the skeletal muscles. The experimental results clearly indicated that the specific radioactivity of the tracer amino acids and the rate of incorporation of radioactivity into tissue proteins were greatly influenced by the size of the free amino acid pool within the range of distribution of the tracer. An estimation of the magnitude of the pool of free amino acids within the distribution range of the tracer can be obtained from the curve pattern for the decline of specific radioactivity of the corresponding free amino acid in the blood plasma. This pool exhibits a high rate of turnover. In all studies made to evaluate in vivo processes of protein synthesis by use of radioactive tracer amino acids it will be particularly important that consideration should be given to the specific radioactivity of the amino acid in the precursor pool for protein synthesis.