The incorporation of labeled methionine into protein by pituitary tissue.

In recent years numerous reports on the uptake of labeled amino acids by the proteins of tissue preparations have appeared. Various amino acids have been used, and the reaction has been shown to occur in tissue slices (l-4), homogenates (5), and in resting bacteria (6, 7). Considerable evidence has accumulated, indicating that this labeling of protein represents a synthesis of peptide bonds. In particular, it has been established that the labeling reaction, with apparently a single exception (8), depends on the simultaneous oxidation of a suitable metabolite, that the label cannot be removed by prolonged dialysis (9), and that, when carboxyl-labeled amino acids were used, the label is not removed by treatment with ninhydrin unless the protein is first subjected to hydrolysis (10). It seems clear that the labeling reaction can be used to measure the over-all activity of the enzymes involved in the synthesis of protein. This report concerns the incorporation of S36-labeled methionine into proteins, rat pituitary tissue being the source of the protein-synthesizing enzymes. It might be expected that this tissue, being specialized in the production of protein hormones, would be highly active in the incorporation of amino acids into protein, and this has been found to be the case. In addition, the effect of various physiological conditions which are known to be related to pituitary function has been investigated. The use of methionine as the labeled substance in such studies has been investigated by Tarver and his coworkers (2, 9). These workers have stressed the error introduced by the conversion of the methionine to cystine and the subsequent labeling of protein by the formation of disulfide bonds (1). In this work this error was eliminated by removal of the mercaptans present in the proteins as the cuprous salts. Thus, only radioactivity incorporated as methionine was determined.