The Use of Radioactive Lysine in Studies of Protein Metabolism

Abstract

Racemic lysine labeled with C14 in the epsilon carbon position was fed to dogs. The distribution of C14 in blood and tissue fractions is recorded. In normal dogs sacrificed at 24 hours, approximately one-third of the C14 was found in the urine, one-third in expired air, and one-third in the body, mostly in protein, predomantly as lysine residues. The rate of C14 excretion as CO2, hour by hour, paralleled closely the amount of non-protein C14 in the blood plasma. The liver, kidney, pancreas, and spleen all have high values for C14 in 24 hour and 17 day experiments. The gastrointestinal tract is significantly high in the 24 hour experiments. Plasma protein from animals previously fed C14 containing lysine and thus in turn labeled, was transfused into other dogs and the rate of disappearance of albumin and globulin fractions from the circulation of the recipient dog followed. The results lead to the conclusion that as a whole, plasma proteins are utilized and replaced at a rate of at least 10 per cent per 24 hours. This minimum rate is substantially faster than turnover rates commonly accepted and emphasizes the rôle played by the plasma proteins in the protein economy of the body. The exact rate determination is made uncertain by the lack of knowledge of the magnitude of the amount of protein in solution in extracellular and lymph spaces and its rate of equilibrium with circulating plasma proteins. Evidence from these transfusion studies indicates that plasma globulin is metabolized at a significantly faster rate than plasma albumin. This is confirmed by the observation that following the feeding of labeled lysine to dogs, C14 is first incorporated in globulin in high concentration but that later it also disappears more rapidly from the globulin fraction. These data suggest that the period of bone marrow maturation of the red cell during which time its related hemoglobin is synthesized does not exceed 3 to 5 days.

Plasma Protein Turnover and Tissue Exchange

“…Details of this procedure, sampling schedules, and methods dealing with the collection and analysis of blood, urine, expired CO2, and tissues were essentially similar to those described in previous publications (5,10,11).…”

The Mechanism of the Immunological Response

“…Experiments with 15N-lysine or with 14C-lysine show that about 50% of the plasma proteins are exchanged within 24 hr. and that the liver is able to produce plasma proteins (Miller, Bale, Yuile, Tishkoff & Whipple, 1949). However, plasma proteins are also formed in other tissues as is proved by their formation in tissue cultures of fibroblasts (Landsteiner Sz Parker, 1940).…”

Albumin and Globulin Circulation in Experimental Ascites

“…As indicated in the individual experimental histories, the amounts of C 14 activity administered as albumin and globulin vary somewhat from experiment to experiment. These variations in donor plasma reflect differences in time of collection after labeled lysine feeding, which ranged from 8 to 17 days, and the number of prior bleedings as related to the previously reported observation that the metabolic turnover of globulin is significantly faster than that of albumin (9).…”

“…The animals incorporated the tagged material in their body proteins and served as donors of radioactive plasma for the test animals. Details of this procedure appear elsewhere (9,10,14). The plasma obtained from bleeding donor animals was separated by centrifugation under sterile conditions and used immediately when administered to the test animal intravenously.…”