Protein Synthesis in Rat Lymphocytes: Radioautographic Studies of Availability and Utilization of Labeled Amino Acids in Vivo

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Abstract
Injections of H3-methionine and H3-leucine were combined with radiochemical and radioautographic technics to study the availability time of H3-methionine and the protein synthetic ability of rat lymphocytes in vivo. Although 98.5 per cent of H3-methionine was removed from the serum 5 minutes after injection, sufficient quantities persisted and/or re-entered the serum from tissues to cause increasing grain counts in radioautographs of large lymphocytes for 1 hour after isotope administration. A small amount of additional labeling occurred during the 2nd hour, but it is calculated that labeling is 97-98 per cent complete by 1 hour. All of the large and medium lymphocytes were labeled in the thymus, lymph node, and thoracic duct lymph at short intervals after injection of 4 µc./Gm. body weight of H3-methionine. Evidence is presented that protein synthesis occurs in the nucleus as well as in the cytoplasm and that newly formed protein is equally distributed between daughter cells following mitosis. Previous immunochemical studies are combined with information on generation time and disappearance rates of radioactivity to suggest that large and medium lymphocytes are constantly producing and releasing proteins. Large and medium cells in lymph and lymph node are more active in this than are similar cells in the thymus. Evidence of reutilization of labeled metabolites in the lymph node and especially in the thymus is discussed. Although not all small lymphocytes were labeled by 4 µc./Gm. body weight of H3-methionine, it was shown that larger doses of isotope would label 100 per cent of them. Small lymphocytes in thoracic duct lymph evidenced significant turnover of labeled protein during the 1st day after isotope administration.