12 JOHN H. NORTHROP 



anism were present which would remove the pentapeptide, the reaction 

 could be used as a source of the substance. In this case the energy for 

 synthesis would be furnished by the mechanism which removed the 

 peptide. 



Synthesis of Proteins from Compounds other than Amino 

 Acids. Several reactions are known (cf. Bergmann and Fruton, 1944) 

 which could give rise to proteins. Alcock (1936) notes that plants can 

 form proteins from sources other than amino acids. He claims with 

 some justification that two entirely different mechanisms for protein 

 synthesis would not be expected and suggests that animals first change 

 the amino acids to the hypothetical building stones which are common 

 to both plants and animals. An "ur-protein" molecule is then synthesized 

 and the other proteins derived from this "ur-protein." This hypothesis 

 is logically attractive but must be amplified somewhat to take into ac- 

 count the energy changes required (cf. page 10). 



If it could be shown that the compounds taking part in these reactions 

 are formed from amino acids by the addition of energy and that these 

 compounds then reacted to form proteins with the liberation of energy, 

 the mechanics of synthesis would be greatly clarified. Synthesis of 

 proteins from such high energy compounds has been discussed by Del- 

 briick (1941) and Gulick (1944). Such a reaction would account for 

 the fact that N or C isotopes supplied in any amino acids are very soon 

 found in all amino acids (except lysin) (Schoenheimer, 1942). This is 

 difficult to understand if the proteins are synthesized from amino acids, 

 but would be expected if amino acids are first changed to other inter- 

 mediates before synthesis occurs. 



At present, however, there does not appear to be any experimental 

 evidence for the existence of such compounds, or reactions, or the neces- 

 sary catalysts in biological material. The enzymatic synthesis of glu- 

 tamic acid from NH3 and the keto acid (Euler, Adler, Giinther, and 

 Das, 1938) is probably the nearest approach to such a reaction so far 

 discovered. Glutamic acid can be converted into several other amino 

 acids by transamination (Braunstein and Kritzmann, 1937; Braunstein, 

 1939). The transmigration of the methyl groups in certain amino acids, 

 studied by DuVigneaud and others (reviewed in Borsook and Dubnoff, 

 1943) furnishes another partial mechanism for equilibria between 

 amino acids. These reactions can account in part for transformation of 

 one amino acid into another but do not supply a complete synthetic 

 reaction for proteins. 



