VITAMIN B 5^ 



Normally aneurin exists chiefly in the reduced (thiol) form milike coenzymes I 

 and II. Its polarographic behaviour is described in Chapter VIII. 



The enzyme carboxylase is probably a compound of aneurin pyrophosphate . 

 magnesium and protein (Green, Herbert and Subrahmanyan, 1940). In bacterial 

 cultures pyruvic acid breakdown appears to follow a different course, but propionic 

 acid bacteria, etc., have been found to synthesise aneurin in deficient media. 



In normal brain tissue carbohydrate is completely oxidised, but Peters has found 

 that in thiamine-deficient pigeons carbohydrate breakdown proceeds only as far as 

 pyruvic acid. These pigeons show convulsive symptoms which are reheved rapidly 

 by administration of thiamine. By tissue phosphorylation thiamine is converted to 

 cocarboxylase and the pyruvic acid accumulation diminishes. Aerobically the 

 pyruvic acid is completely oxidised, but anaerobically it is decarboxylated oxidatively 

 to acetic acid whilst a second molecule is reduced to lactic acid, coenzyme I carrying 

 out the necessary hydrogen carrying : — 



Pyruvic acid + HgO + Col -> Acetic acid + COg + CoI.2H 



Pyruvic acid + CoI.2H -> Lactic Acid + Col. 



The use of the pyruvate oxidase system in the investigation of vesicants is 

 described in Chapter VII. 



The mechanism of oxidative decarboxylation by B. delbruckii has been studied 

 by Lipmann, who traced the intermediate formation of acetyl phosphate. A fuller 

 discussion of energy-rich phosphate bonds is reserved for the next Chapter, but it is 

 sufficient to mention here that in acetyl phosphate, adenosine diphosphate (ADP) 

 and adenosine triphosphate (ATP) the pyrophosphate groups are in a peculiarly 

 reactive state indicated by the sign r--'ph. The reaction proceeds : — 



Pyruvic acid + phosphate -> Acetylphosphate + COg 



CH COOr^ph + ADP -> CH3COOH + ATP 



The w^hole process is evidently complicated and cocarboxylase, coenzyme I,. 

 adenosinediphosphate in addition to inorganic phosphate and magnesium ions are all 

 participants in oxidative decarboxylation of pyruvic acid. 



VITAMIN Be, PYRIDOXAL PHOSPHATE 



Vitamin Bg, an antidermatitis vitamin, is pyridoxine and the related compounds- 

 pyridoxal and pyridoxal phosphate play most important roles in amino acid decar- 

 boxylation and in transamination. 



The structure of pyridoxal phosphate is probably : — 



CHO 



^N 



'3^2 



P yridoxal phosphate 



Related compounds function as coenzymes in the decarboxylation of a-amino- 

 acids as the prosthetic group of transaminases, occasionally pyridoxamine phosphate 



