Cellular oxidations and the synthesis of amino-acids and amides in plants 



amination of a-ketoglutaric acid, followed by further combination with ammonia 

 to give the y-amide. The course of the synthesis is outlined below. 



H 2 CoI 



Col 



ATP 



a-ketoglutarate 



Transamination 



+NH, 



Glutamate 



ADP 



+NH 2 



Alanine (ioo) 

 Aspartic acid (55) 

 Isoleucine (12) 

 Leucine (5) 

 Valine (5) 

 Glycine (1) 



Transamidation 



Glutamine 



Some evidence of the occurrence of these reactions in barley seedlings has been 

 gained by the separation of enzymes from the young embryos. Highly active pre- 

 parations of glutamic acid dehydrogenase, which catalyses the reductive amination 

 of a-ketoglutarate linked with the oxidation of pyridine nucleotide (Col), have been 

 obtained from the seedlings. With yeasts similar preparations of the dehydrogenase, 

 but reacting with coenzyme II, have been obtained by Adler and others (1938), 

 while Elliott (1951) has demonstrated the enzymic synthesis of glutamine coupled 

 with a conversion of adenosine triphosphate (ATP) to adenosine diphosphate (ADP). 



The dependence of amide synthesis on phosphorylation has been further indicated 

 in the present experiments by the action of 2.4-dinitrophenol. This cell poison 

 strongly inhibits the synthesis of glutamine at low concentrations (0-6-2 -5 x io -5 m 

 at pH 55), although at these levels the rate of oxygen uptake is unaffected, or slightly 

 increased. This typical uncoupling action is attributable to the selective action of 

 dinitrophenol on the phosphorylations linked with cellular oxidations (Simon, 1953). 



The enzymic systems engaged in the synthesis of glutamine may readily account for 

 its close co-ordination with carbohydrate metabolism and respiration. a-Ketoglutaric 

 acid, the organic acid precursor, is an intermediary in the oxidation of carbohydrate 

 by the tricarboxylic acid cycle, while pyridine nucleotides and adenosine triphosphate 

 occupy key positions as electron and phosphate carriers respectively in cell oxidations. 

 These direct links with the exergonic reactions of respiration may form the starting- 

 point in the synthesis of other amino-acids and of peptides by transfer reactions, such 

 as transamination and transamidation, which proceed with relatively little change 



59 



