CENTRAL NERVOUS SYSTEM METABOLISM IN VITRO 



1833 



metabolism of uniformly labelled C 14 glucose gave 

 rise to labeled glutamic and aspartic acids within 2 to 

 4 min. of incubation, followed by incorporation into 

 7-aminobutyric acid (14). After incubation for 1 hr. 

 in a phosphate buffered medium, 9 per cent of the 

 glucosed metabolized was present as glutamic acid, 

 3 per cent as 7-aminobutyric acid and 2.4 per cent as 

 aspartic acid. The remainder was accounted for as 

 lactic acid and carbon dioxide. No other labeled 

 amino acids except a trace of alanine were detected. 

 Formation of these amino acids may be the starting 

 point for the synthesis of other amino acids and 

 possibly proteins of the central nervous system. Thus 

 in vitro experiments with minced brain from 1 -day-old 

 mice showed that in 24 hr. the carbon of uniformly 

 labeled C H -glucose was distributed in all the amino 

 acids of cerebral proteins with the exception of proline 

 and threonine. On the basis of previous work (169) 

 the authors considered that such labeling could not 

 be due to incorporation of radioactive carbon dioxide 

 produced during the experiment, but the evidence 

 provided in support of this view is not wholly con- 

 vincing. 



glutamic acid. Since the role and importance of 

 glutamic acid in the metabolism of the central nervous 

 system has been the subject of several recent reviews 

 (136, 209, 210, 214, 219, 220), comment here will be 

 restricted to major points and to recent developments. 

 Glutamic acid together with its amide glut amine 

 constitute up to 80 per cent of the free a-amino nitro- 

 gen of cerebral tissues. Slices of cerebral cortex in a 

 phosphate or bicarbonate buffered saline were able to 

 maintain a concentration gradient of glutamic acid 

 provided glucose was present in the saline as an en- 

 ergy-yielding substrate (191). Other substrates such as 

 fructose, lactate or pyruvate also assisted accum- 

 ulation but were not as effective as glucose, Glu- 

 tamate alone could not maintain the tissue con- 

 centration of glutamic acid; neither could anaerobic 

 glycolysis unless adenosine triphosphate was added to 

 the medium. In slices, homogenates or in particulate 

 preparations, glutamate has been shown to undergo 

 a variety of reactions which include: a) conversion to 

 glutamine (44, 101, 102, 1 13, 191 ); />) decarboxylation 

 to yield 7-aminobutyric acid (172, 209); c) oxidative 

 deamination to yield a-oxoglutaric acid and ammonia 

 (30, 34, 216); d) transamination principally with 

 oxaloacetic acid (28) but also with other a-keto acids 

 (173); e) exchange of the amido group with other 

 amines in the glutamotransferase reaction (105, 181), 

 a process requiring energy in the form of adenosine 



triphosphate; and /) transpeptidation (49, 70) 

 whereby the glutamyl group of glutathione is trans- 

 ferred to other amino acids, thus forming glutamyl 

 peptides. The multiplicity of reactions undergone 

 assigns to glutamic acid a place of importance in the 

 central nervous system, both as a means of controlling 

 the intracellular concentration of ammonia and 

 possibly as a source of amino groups in the synthesis 

 of protein. 



Much recent work concerns the production, pos- 

 sible function and fate of 7-aminobutyric acid In the 

 animal body this amino acid is found in appreciable 

 concentration almost solely in nervous tissue (175) 

 and is formed at a rate, calculated from the data of 

 Roberts & Frankel (174), reaching 30 pinoles per 

 gm wet wt. tissue per hr. under optimal conditions. 

 It has been shown (13, 42) that 7-aminobutvric acid 

 is identical with a factor, extracted from brain, which 

 reversibly blocks stretch receptor neurons of the cray- 

 fish. 7-Aminobutyric acid was shown to be syn- 

 thesized and to exist in brain in a form not available 

 for tins reaction (called by Klliott and collaborators 

 an 'occult' form), but is released l>\ boiling in water, 

 by dilute hydrochloric acid or by alkali. It is sug- 

 gested thai 7-aminobutyric acid may be the trans- 

 mitter substance of inhibitory neurons. Such a role 

 is in keeping with its existence in an 'occult' form and 

 requires also that the acid be metabolized to less 

 active products. In this latter connection the major 

 metabolic route probably is transamination with 

 a-oxoglutarate to form succinic semialdehyde and 

 glutamic acid (15, 1 7 ; I 



AMMONIA FORMATION. As mentioned above one func- 

 tion of the glutamine-glutamic system probably is 

 the removal ol excess ammonium ions from the tissue, 

 a fact which raises the problem of the origin of such 

 ammonia. Cerebral slices, when incubated in a phos- 

 phate or bicarbonate saline in the absence of sub- 

 strate, evolve ammonia over a period of several hours 

 (219, 223) without an\ signs of the production coming 

 to an end. Ammonia formation is inhibited by anaer- 

 obic conditions or by inhibitors which affect oxidative 

 phosphorylation or electron transport. Production is 

 inhibited also by glucose. 



Attempts to assign such formation to the activity 

 of nucleoside and nucleotide deaminases (92, 152, 

 222, 223) have shown that this route of formation is 

 unlikely, particularly in view of the relatively small 

 amounts of nucleotides present in the tissue. The 

 other known deamination systems of cerebral tissue, 

 v iz. glutaminase, glutamic dehydrogenase and amine 



