The utilization of labeled glycine in the process of amino acid incorporation by the protein of liver homogenate.

The incubation of rat liver slices in an oxygenated medium in the presence of either S35-labeled methionine (1) or C4-labeled alanine (2) results in an uptake of the radioactive amino acid into the protein of the slices. A similar react,ion has been noted between the.protein of intestinal slices and C’Jlabeled glycine (glycine*) (3). Presumably the residues of the labeled amino acids become components of the protein molecules. The inhibition of the Cl4 incorporation by heat or sodium azide suggests that the process is mediated by enzymes (3). Recently (4) it was reported that glycine” is incorporated into the proteins of homogenized preparations of spleen and liver upon incubation under suitable conditions. Further work has shown that the process involved is not simply one of glycine” incorporation into protein. Instead, the major portion of the Cl4 found in the homogenate protein can be attributed to other amino acids, particularly serine, derived apparently from glycine. These results are reported separately (5). The present paper describes the effect of various metabolites, inhibitors, and certain experimental conditions on the process of Cl* incorporation by liver homogenate protein in the presence of glycine*. Of several different organs tested, rat spleen yielded the most active homogenate preparations. However, spleen tissue is complex histologically, in that it consists of several types of small cells, and the destruction of the latter by mechanical grinding is difficult to gage. By contrast, liver consists mainly of cells of one type. These cells are rather large (about 25 to 30 iv. in diameter in the rat) and are readily ruptured in a glass homogenizer. The small fraction of the hepatic cells which escapes disintegration can be removed by low speed centrifugation. Homogenat.es prepared in this manner are free from intact liver cells. They contain abundant nuclei, as well as cytoplasmic material, leucocytes, and erythrocytes.