Y-GLUTAMYL PEPTIDES IN PLANTS 63 
It has been suggested that transpeptidation might be involved in protein synthesis. 
Glutathione and other y-glutamyl peptides are not apparently involved in protein 
synthesis*®. Since several of the naturally occurring y-glutamyl peptides contain 
amino acids not found in proteins (S-methyleysteine, f-alanine, /-aminoisobutyric 
acid and hypoglycin A), a specific role in protein synthesis seems unlikely. 
The y-glutamyl peptides have been found primarily in storage organs and constitute 
an appreciable proportion of the non-protein nitrogen. Since most of the nitrogen of 
bulbs and seeds occurs as proteins (80-90%), storage of nitrogen as peptides is not 
reasonable unless they are special storage forms for the non-glutamic acid portion 
of the peptide. 
The data in Table III indicate that y-glutamylmethylcysteine may be transported 
from the cotyledons to other parts of the plant. Dipeptides may act as transport forms 
of nitrogen, glutamic acid or the non-glutamic acid portion. Because of its central 
role in nitrogen metabolism, glutamic acid would be an excellent source of nitrogen 
for any tissue. It is equally possible that the non-glutamic moiety is transported in 
this way or that the peptide form protects the glutamic acid from being metabolized 
in the conductive tissue before it reaches active metabolic regions. In the onion plant 
peptides are not found in tissues other than the bulb suggesting that translocation 
of intact peptides does not occur. 
Since the non-glutamic acid portions ($-aminopropionitrile, p-hydroxymethyl- 
phenylhydrazine, methylcysteine, p-hydroxyaniline) of several y-glutamyl compounds 
are unusual groups, they may be toxic to the plant. Combination with glutamic acid 
might well reduce toxicity. However, this concept would not fit the case where normal 
protein amino acids are bound. It is, of course, equally puzzling as to why the plant 
synthesizes /-aminopropionitrile and p-hydroxymethylphenylhydrazine. 
The discovery of y-glutamyltyrosine and y-glutamylphenylalanine in soy bean meal 
may have some bearing on the toxicity of soy bean meal. BorcHERs has found?’ that 
the toxicity of raw soy bean meal is reduced by tyrosine and not by phenylalanine. 
Possibly these peptides interfere with the hydroxylation of phenylalanine to form 
tyrosine. 
It may well be that the various dipeptides have different functions in the diverse 
places where they occur. 
Poly-y-glutamyl peptides also occur in nature*®-*°, These appear to have a different 
metabolism from the aforementioned y-glutamyl compounds. 
ADDENDUM 
Since the time that this manuscript was submitted, VIRTANEN AND MATILLA®! 
reported the isolation of y-glutamyl-S-allyl-L-cysteine from garlic. Also, VIRTANEN 
AND MATIKKALA®™ have crystallized from onion three more y-glutamyl peptides; 
y-glutamylmethionine, y-glutamyl-S-methylcysteine and y-glutamylleucine. Further 
proof of the identity of y-glutamylleucine from kidney beans has been obtained by 
comparison of the infra red spectra of the isolated and synthetic peptides. The spectra 
were identical. The new peptides indicate that one can expect that many more y-glu- 
tamyl peptides will be found. 
References p. 64 
