REACTIONS OF CARBON DIOXIDE 99 



Glutamic acid decarboxylase. This enzyme is widely dis- 

 tributed in plants and has been found both in green and in 

 non-green tissues. Pyridoxal phosphate is required as a 

 coenzyme, the reaction catalysed being, 



COO-.CHNH3+.CH2.CH2.COO-+H+ - 



CO2+CH2HN3+CH2.CH2.COO- 



Glutamate y-Aminobutyrate 



The product of decarboxylation, y-aminobutyric acid, has 

 been shown to be present in extracts from a variety of plant 

 tissues. The glutamic decarboxylase from Clostridium 

 zvelchit has been shown to cause an incorporation of ^^COg 

 into a carboxyl group of glutamic acid by Mandeles and 

 Hanke. Reversibility has not been studied in the plant. 



Decarboxylation of pyruvate by carboxylase. The formation 

 of COo from pyruvate in alcoholic fermentation is catalysed 

 by the enzyme carboxylase which was discovered by 

 Neuberg; the decarboxylated product acetaldehyde is then 

 reduced to ethanol. There is no evidence that this decar- 

 boxylation is reversible. Auhagen, however, discovered that 

 carboxylase could be resolved into cocarboxylase and a 

 protein component (apocarboxylase). Cocarboxylase was 

 shown by Lohman and Schuster to be the pyrophosphate 

 of vitamin B^ (thiamine) and the work of Peters and col- 

 leagues showed that cocarboxylase was essential for the 

 oxidative metabolism of pyruvic acid in the animal. This 

 coenzyme is also required for the oxidative decarboxylation 

 of alpha-ketoglutaric acid. It has been proved that vitamin 

 Bj is essential for the growth of plant embryos when they 

 are deprived of their cotyledons and supplied with carbo- 

 hydrate. 



The oxidative decarboxylation of pyruvate. The simplest 

 equation for this process would be: 



CH3— CO— COO-+2H0O-2H - CH3— COO-+HCO3-+H+ 

 Pyruvate Acetate 



It was found however that the formation of acetyl choline 

 by tissue preparations from brain was dependent on the 



