H. WAELSCH 
NI 
bo 
C 
appears that brain cortex has a potent CO, fixing mechanism. It is interesting to 
note that when NH, is administered together with the [14C|bicarbonate, there is a 
shift in counts to glutamine, as expected from the fact that glutamic acid synthesis 
is increased in order to remove ammonia by glutamine formation (Table V). Although 
the final demonstration of CO, fixation in the brain is, of course, not dependent on 
the compartments of metabolism, the proof that cerebral glutamine cannot be 
derived from the blood was facilitated by the demonstration of a compartment of 
glutamine synthesis. It may be suggested that an understanding of metabolic pools 
and compartments is sometimes essential for a critical interpretation of metabolic 
events. 
What are the particular characteristics of the experiments which provided the 
TABLE V 
LABELING OF GLUTAMIC ACID, GLUTAMINE AND ASPARTIC ACID 
AFTER INFUSION OF AMMONIUM ACETATE AND [!4c|]NaHCOg 
Infusion: 25 mmoles ammonium acetate; 0.2 mmole [14C)|NaHCO 
5 Tage 3 
0.65 mC in 25 min. 

Counts/min] umole 



Compound —— — — — 
Blood Brain cortex Liver 
Glutamine i 180 830 300 
Glutamic acid 210 290 600 
Aspartic acid 150 370 1340 

results discussed in this presentation? What are the implications of these results for 
an understanding of intermediary metabolism as far as turnover rates, control of 
metabolism and cerebral functions are concerned? The experiments presented are 
characterized by their short duration, none lasting longer than 60 min. Therefore, 
the initial fate of the labeled intermediate is studied before the isotope has penetrated 
all metabolic pools. In such experiments fast reactions will be measured; in long- 
time experiments some of the rapidly metabolized compounds will have lost their 
label, and mainly the metabolism of compounds with long half-life times will be 
estimated. Since in experiments of long duration some of the pools will have been 
penetrated by the isotope precursor and its metabolic derivatives, usually the 
precursor product relationship of the classical isotope experiments will be found. 
Turnover values of metabolites are the sum of the turnover values of the meta- 
bolites in different metabolic compartments. Therefore, any overall turnover value 
of a tissue constituent has only limited biological significance. This conclusion, a 
clear consequence of the existence of metabolic compartments, becomes of particular 
importance if intermediary metabolism is to be related to the function of an organ. 
It is apparent that the glutamic acid with a half-life time of several hours will have a 
different functional involvement than that with a half-life of less than 1h. The 
former may serve as a relatively stable cellular anion. In the experiment presented, 
the rate of conversion of the metabolite was the indicator of the existence of a pool. 
From the comparative rates conclusions were drawn as to the enzymatic make-up 
References p. 730 
