W. YEMM 



of free energy. Steward and Street (1946, 1947), Yemm (1949), Fruton (1950), 

 Hanes et al. (1950), and Waelsch (1952) have discussed the potentialities of glut- 

 amine in the canalizing of energy to protein synthesis. 



Transaminases, which promote the transfer of a-amino groups from glutamic acid 

 to other a-keto acids, are known to be widely distributed in higher plants (Leonard 

 and Burris, 1947) and their presence in food yeast was demonstrated by Roine (1947). 

 A preliminary investigation of these enzymes in young barley seedlings has shown that 

 they provide a mechanism for formation of at least six other amino-acids, as indicated 

 above. Estimates of the relative rates of transamination with the different amino- 

 acids are given in the diagram. It is of interest that the glutamic-alanine and glut- 

 amic-aspartic systems give the highest activities, which may account for the forma- 

 tion of alanine and asparagine during the rapid assimilation of nitrogen. 



The nature of the transamidation reactions and their significance in the biosyn- 

 thesis of peptides and proteins is at present uncertain. However, Dowmont and 

 Fruton (1952) have found that plant proteinases, such as papain and ficin, catalyse 

 the synthesis of peptide bonds from amides by transamidation, so that, in artificial 

 systems, formation of polypeptide structures occurred. Participation of the y-amide 

 group of glutamine in transfer reactions in the cell is indicated by the occurrence of 

 glutamyl transferase in micro-organisms (Grossowicz, Wainfan, Borek and Waelsch, 

 1950) and in higher plants, (Stumpf, Loomis and Michelson, 1 951). In this connexion 

 preparations of glutamyl transferase have recently been made from barley seedlings 

 and the activity estimated in model systems by measuring the rate of replacement of 

 the amide group of glutamine by hydroxylamine. The activity of the enzyme in cell- 

 free preparations indicates that it could play a substantial part in peptide synthesis : 

 the rate of transfer of amide groups observed in cell-free preparations is, in fact, 

 adequate to account for the high rates of peptide synthesis which occur in the young 

 embryo. 



The products of the action of y-glutamyl transferase in the cell are not yet known. 

 The work of Hanes and others (1950, 1952) has suggested that the tripeptide, gluta- 

 thione, which is very widely distributed in living cells, may take part in transpeptida- 

 tions involving the transfer of y-glutamyl groups. But, under the conditions so far 

 tested, the tripeptide is inactive with the glutamyl transferase of barley and, in yeast, 

 the changes of glutathione during assimilation of nitrogen are relatively small, as 

 already indicated. On the other hand, there is some evidence that the formation of 

 glutathione may be correlated with protein synthesis in the early stages of the 

 development of barley embryos. Estimated by means of the nitroprusside reaction of 

 Grunert and Phillips (1951), the peptide increases markedly at a time when syn- 

 thesis of protein is beginning, as shown by the results given in Figure 6. 



Mainly in the reduced form, glutathione accumulates in the tissues after about 

 two days' germination and at the same time there is an acceleration of protein syn- 

 thesis. Histochemical tests indicate that it occurs mainly in the meristematic regions, 

 which are in all probability very active in the synthesis. However, it is possible that 

 the action of the tripeptide in oxidation-reduction systems of the cell, recently 

 elucidated by the work of Conn and Vennesland (1951) and Mapson and Goddard 

 (1951), may account for this relation. Moreover, glutathione represents only a very 

 small part of the total soluble nitrogen of the embryo, and other analyses suggest the 



60 



