72 CHAIRMAN: E. ROBERTS 
interesting side note here, that EvANs AND Tsar in my laboratory fed radioactive thymine to root 
sections of the Wedgewood strain of iris and isolated 6-aminoisobutyric acid. Of course, it would 
have been quite surprising to have found it otherwise. 
TuHompson: Yes. Of course, we more or less assumed it came from thymine. 
E. Roperts: HANeEs’ early work with the y-glutamyl peptides suggested the possibility that 
these compounds might be related to important metabolic sequences in living cells. Would some- 
body care to hazard a guess as to what role such compounds, besides glutathione and glutamine, 
might be playing in the metabolism of cells, in protein synthesis, or other processes. 
THompson: Well, of course, we have thought about this quite a bit. In our paper we suggested 
that they may have something to do with storage and with translocation, but we really do not have 
any evidence. The occurrence of such large quantities of these compounds certainly is very in- 
triguing. We have seven or eight y-glutamyl compounds, and it appears that many more may be 
found. At present, one can only surmise that their occurrence in these rather large quantities 
indicates that they play a role in normal metabolism. On the other hand, Hrrp suggested that 
the only reason he could see for the occurrence of the y-glutamyl transpeptidase, which is the 
enzyme that Hanes originally reported on in 1950, was that it provides employment for bio- 
chemists. 
HENDLER: I might mention that I spent a couple of years working on the significance of y-gluta- 
myl peptides in protein biosynthesis in Dr. GREENBERG’s laboratory. We synthesized y-glutamyl 
glycine labeled with radioactive glycine, and compared the rate of uptake of free glycine and 
glycine, so bound, into protein in about five or six different protein synthesizing systems. In every 
case free glycine was much more efficiently taken into the protein than y-glutamyl peptide bound 
glycine. In no case did we ever find an uptake of radioactive amino acid into y-glutamyl peptides 
when labeled amino acids and various unlabeled y-glutamyl compounds supplied as donors of the 
y-glutamyl group were incubated together. One other point that was advanced favoring the partici- 
pation of y-glutamyl! peptides in protein synthesis was their resistance to hydrolysis by cellular 
peptidases. However, we found them to be quite readily hydrolyzed. We found no evidence 
whatsoever that y-glutamyl peptides played a role in amino acid incorporation in the variety of 
systems that we studied. 
E. Roperts: I wonder if there has been any evidence that the y-glutamyl group can be activated 
by amino acid-activating systems. 
LoFTFIELD: TuBor AND HuziNno? in Japan found activation of a number of peptides by amino 
acid-activating enzymes. I do not know whether they included any glutamyl peptides. Other than 
that there is, of course, the synthesis of glutathione, but this is quite a different process from the 
ordinary amino acid-activating system. 
E. Roperts: I still have a question about the problem that Dr. Srewarp brought up with regard 
to the synthesis of y-aminobutyric acid from succinic semialdehyde as a natural pathway in plants. 
I am willing to be convinced, but it does seem to me that there is, at present, no known metabolic 
source of succinic semialdehyde that has been proven. Of course, succinic semialdehyde could 
come from a-ketoglutarate if the oxidase system were not working properly. In in vitvo experi- 
ments using preparations from /animal tissues, the formation of succinic semialdehyde from 
a-ketoglutarate has been shown 'to take place only when the a-ketoglutarate oxidase system has 
been partially inactivated. 
REFERENCES 
1 A. I. VIRTANEN AND P. K. Hretara, Acta Chem. Scand., 9 (1955) 175. 
2 L. Fowpen, Aun. Rep. Progr. Chem., 56 (1959) 359. 
3S. Tusor anp A. Huzino, Arch. Biochem. Biophys., 86 (1960) 309. 
