52 INTERMEDIARY METABOLISM AND GROWTH 



(CH2)2 



CH NH2 



COOH 



■+- CH,— CO-COOH 



(CH2)2 



CO 

 COOH 



+ CH,— CH— COOH 



NH; 



-*i^H, + 



COOH 



(CH2)2 



CO 



I 



COOH 

 Fig. 39. Glutamine as a transaminating agent. 



are intermediates in amino acid synthesis (Silver and McElroy, 1954). As shown 

 in Fig. 38, four inutants were isolated which lacked nitrate reductase and 

 required nitrite for growth (Block I, Fig. 38). Eight NH3 requiring strains (Block 

 II, Fig. 38) were also found. Most of these contained the enzymes, nitrite reductase and 

 hydroxylamine reductase although two strains accumulated nitrite and hydroxylamine in the 

 medium. One strain (Block IV, Fig. 38) required pyridoxal for growth. If pyridoxal 

 was lacking in the medium, nitrite accumulated and nitrite- and hydroxylamine- 

 reductase were reduced in concentration. In the presence of pyridoxal, the mutant 

 grew on nitrate or nitrite suggesting that vitamin Bg is directly involved in nitrite 

 utilization. Strains blocked at III also require pyridoxal and accumulate nitro- 

 phenyl hydroxylamine when tested for the ability to reduce dinitrobenzene. 

 Strains blocked at II do not accumulate nitrophenylhydroxylainine. 



The possibility that oxime compounds are intermediates in amino acid synthesis is also 

 suggested by experiments with Torulopsis (Virtanen and Saris, 1955). Torulopsis cells 

 grown in the presence of nitrite or nitrate generate bound oxime derivatives of glyoxylate, 

 oxalacetate, pyruvate, and a-ketoglutarate. Upon chemical reduction, glycine, aspartate, 

 alanine, and glutamate, respectively are formed. 



2. NH^ fixation 



Enzymatic reactions resulting in the fixation of NH, are widely distributed in 

 living cells. At the present time, seven ammonia fixation reactions may be dis- 

 tinguished : 



(j) Glutamic dehydrogenase. The enzyme, glutamic dehydrogenase, which occurs 

 in liver, kidney, muscle, brain, and heart, catalyzes the following reaction: 



TPNH2 + NH3 + a-ketoglutarate - — r gkUamic + TPN"" 



The concentration of the enzyme is several fold higher in leukemic than in normal 

 leukocytes (Waisman et al., 1956). 



(2) Glutamine synthetase. The synthesis of glutamine from glutamic acid and NH3 

 requires Mg"^"^ and ATP. Glutamine synthetase occurs in liver, kidney, muscle, 



