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Danilewski described the formation of a protein-like substance when pepsin was added to a concentrated peptic digest of proteins. The subject has since been carefully investigated by a number of workers, especially Henriques and Gjaldb~ck and Wasteneys and Borsook. The latter authors have reviewed the work in detail (Wasteneys and Borsook, 1930). There is no doubt that some sort of a synthetic reaction occurs since the number of free amino and caxboxyl groups decreases and at the same time a substance appears in solution which is heat-coagulable, is precipitated by trichloracetic acid, and has other protein characteristics. The relation of this substance to the protein from which it is derived, however, is not so definite. Robertson considered that plastein formed from hydrolyzed casein solution was paranuclein, a compound closely related to casein. Immunological tests confirmed this conclusion. A .E . Taylor reported that a protamine synthesized by means of an'enzyme preparation from snails was identical with the protamine used for hydrolysis. The relation of the plastein to the original protein is of interest since, if the reaction is of biological importance in connection with protein synthesis, it is necessary that specific proteins be formed. If, on the other hand, the reaction proceeds simply because an insoluble compound happens to form which then precipitates and so causes the reaction to proceed further, no special relation would be expected between the plastein and the original protein. This point may be tested by using enzyme proteins as the starting point. If any of the original protein is then synthesized during the plastein reaction, its presence could be detected by means of the enzymatic activity. Solutions of pepsin were allowed to autolyze at pH 1.6 and 55°C. under which conditions the protein is hydrolyzed. The digest was then concentrated, adjusted to pH 4.0, and allowed to stand at 40°C. The solution set to a solid paste mixed with tyrosine crystals and a marked increase in substance precipitated with trichloracetic acid occurred. No increase in peptic activity, however, could be detected. The plastein formed from hydrolyzed trypsin solutions likewise failed to show any tryptic activity. I t appears, therefore, that the plastein formed is not identical with the protein originaUy hydrolyzed. 377

PLASTEIN FROM PEPSIN AND TRYPSIN