EFFECTS ON AMINO ACID AND PROTEIN METABOLISM 153 



on succinate oxidation, and at these high concentrations this may not be 

 true. On the other hand, Das and Roy (1961, 1962) claim that transamina- 

 tion contributes little to the metabolism of glutamate in mitochondria from 

 Vigna sinensis, and since the decarboxylase is absent, oxidation by gluta- 

 mate dehydrogenase would seem to be the major route. Glutamate is con- 

 verted primarily to aspartate in rat brain homogenate via the pathway 

 glutamate -^ a-ketoglutarate -^ succinate -^ oxalacetate -^ aspartate 

 (Haslam and Krebs, 1963). The addition of fumarate removes this inhi- 

 bition, as expected. 



Certain amino acids appear to be involved in the functioning of nerve 

 tissue and the effects of inhibitors on the metabolism of these substances 

 are of particular interest in this connection. Glutamate is accumulated in 

 brain and plays a role in the active transport of ions, while y-aminobutyrate 

 and A^-acetylaspartate have recently attracted attention because of their 

 ability to modify central nervous system activity. Glutamate and K"*" are 

 taken up by retina and brain slices in approximately equivalent amounts. 

 Malonate at 20 raM depresses the formation of glutamate + glutamine 

 only 12% while it reduces K+ uptake 40% (Terner et al., 1950), indicating 

 that the major effect of malonate is not mediated through interference with 

 glutamate. When guinea pig brain slices are incubated with glucose-u-C^*, 

 a good deal of the C^'* appears in amino acids, the most important of which 

 is glutamate (Tsukada et al., 1958). Malonate at 10 mTlf inhibits glutamate 

 formation around 25%, ^-aminobutyrate formation around 75%, and the 

 formation of aspartate appreciably. The total C^* incorporation into amino 

 acids from glucose-u-C^* in rat brain slices is inhibited 64% by 10 mM 

 malonate at normal K+ concentration but 83% in the presence of 105 

 mM K+, which produces an activation of brain metabolism (Kini and 

 Quastel, 1959). Such results can be readily explained on the basis of a mal- 

 onate-reduced pool of amino acid precursors due to the reduction in cycle 

 activity. Glutamate is a central substance in amino acid formation through 

 transaminations and anything which decreases the formation of a-keto- 

 glutarate would be expected to impair these pathways. Cremer (1964) has 

 recently found that 40 mM malonate not only reduces drastically the 

 incorporation of glucose-u-C^* into glutamate, aspartate, y-aminobutyrate, 

 and protein in brain slices, but also causes a loss of amino acids from the 

 cells. This concentration, of course, is probably not specifically inhibiting 

 succinate oxidation. A disputation type of reaction occurs in certain 

 tissues: 



2 a-Ketoglutarate + NH3 + ADP + P, ^ glutamate + succinate + CO, + ATP 



Tager (1963) used malonate to block succinate dehydrogenase and surpris- 

 ingly found that it augments the formation of glutamate in suspensions 

 of rat liver mitochondria (see accompanying tabulation). It was suggested 



