54 
INVITED DISCUSSION 
y-GLULAMYE, PEPTIDES IN BEANS 
JOHN F. THOMPSON, CLAYTON J. MORRIS, 
WILFRED N. ARNOLD anp DONELLA H. TURNER 
U.S. Plant, Soil and Nutrition Laboratory, Soil and Water Conservation Research Division, 
Agricultural Research Service, U.S.D.A., and the Botany Department, Cornell University, 
ithaca, NEY. (UCS AC) 
Since 1958, when the first y-glutamyl dipeptide was found in legume seeds? *, nine 
additional y-glutamyl dipeptides and a y-glutamy] tripeptide have been isolated from 
plant tissues*-8. Also, there is chromatographic evidence for two more such dipep- 
tides’: 9, and chemical proof for other y-glutamyl compounds in which the non- 
glutamyl portion is a nitrogen containing compound but not an amino acid??™. 
This paper discusses the evidence for the identification of y-glutamyl compounds 
from plants including the proof for two dipeptides which have not previously been 
reported. Determination of the content of y-glutamyl peptides and hydrolytic products 
in various tissues has demonstrated that peptides may account for a large percentage 
of the non-protein nitrogen and that the peptide form of an amino acid may be many 
times higher than that of the free form. Evidence was found for the disappearance 
of one peptide during the germination of kidney bean seeds. Preliminary data for a 
y-glutamyl transpeptidase indicates a possible pathway for the synthesis and break- 
down of y-glutamyl] peptides. 
EVIDENCE FOR THE OCCURRENCE OF y-GLUTAMYL COMPOUNDS IN PLANTS 
Although peptides are to be emphasized in this discussion, other substances in which 
a non-amino acid nitrogenous compound is attached to the y-carboxyl of glutamic 
acid are included because of their obvious similarity to peptides. Except for gluta- 
thione and glutamine there is no evidence for the existence of y-glutamyl com- 
pounds in animal tissues. With the exception of glutamine and glutathione, the 
natural occurrence of y-glutamyl compounds has been known for less than a decade. 
It is therefore pertinent to examine the evidence for their identification. 
y-L-Glutamyl-S-methyl-L-cysteine. This dipeptide has been isolated from Lima bean 
seed (Phaseolus limensis) by buffer chromatography on a cation exchange resin® and 
from kidney bean seed (Phaseolus vulgaris) by dilute acetic acid chromatography on 
an anion exchange resin! 2. In both cases the elemental analyses agreed closely with 
the theoretical values and the hydrolytic products were identical with L-glutamic 
acid and S-methyl-r-cysteine. y-Glutamyl-S-methylcysteine was prepared by the 
methylation of reduced glutathione and action of carboxypeptidase on the resultant 
methylated glutathione. This preparation had an identical infrared spectrum to that 
of the material isolated from kidney bean seeds?. Chromatographic evidence for this 
peptide in other bean seeds has been reported’. 
References p. 64 
