726 H. WAELSCH 
glutamine formation from administered glutamic acid is also true for glutamine 
formation from glutamic acid synthesized in the tissue, namely, that this biosyn- 
thetic event occurs in a metabolic compartment. Since, according to fractionation 
studies, glutamic dehydrogenase is localized in the mitochondria!, one might 
suggest that the glutamic acid newly formed from ketoglutarate and labeled NH, 
TABLE III 
LABELING OF GLUTAMIC ACID, GLUTAMINE AND ASPARTIC ACID 
DURING [!°N]AMMONIA INFUSION (CAT) 
All values represent ®N atom % excess 





Blood Brain cortex Liver 
Compound — - - - = 
I5 min 20 min I5 min 20 min I5 min 20 min 
Glutamine amide 12 14 35 45 38 26 
a-Amino 2 3 9 13 9 6 
Glutamic acid 
a-Amino 5 IO Om7, 1.6 28 35 
Aspartic acid 
a-Amino — — 0.3 1.0 48 46 

is converted in those particles into glutamine before it mixes with the tissue glutamic 
acid. It is of interest to compare these data with those obtained from liver in the 
same experiments. In liver, the a-amino group of glutamic acid always contains a 
higher isotope concentration than the a-amino group of glutamine, contrary to the 
situation in brain. This finding suggests that glutamine in the liver is not formed 
BRAIN LIVER 
MT MT 
o DS 
5 
| 
© © 6 
Fig. 2. Pools of glutamic, aspartic acids and glutamine in brain and liver (15NH,). 
For abbreviations see p. 724. 
from a small compartment of rapidly metabolized glutamic acid but rather from the 
total tissue glutamic acid or a large portion of it. On the other hand, the isotope 
concentration of the a-amino group of aspartic acid soon overtakes that of the a- 
amino group of glutamic acid, while in brain the latter contains an isotope concen- 
tration considerably higher than that of aspartic acid. These findings suggest that 
aspartic acid in liver is formed by transamination of a glutamic acid newly syn- 
thesized, while in brain it is formed by transamination with the total tissue glutamic 
acid or a large part thereof. A tentative picture of this relationship is given in Fig. 2. 
It is interesting in this context to consider the possibility of enzyme mapping by a 
References p. 730 
