CONCLUSION 



The conversion of jsocitric acid into oxalosuccinic acid involves 

 triphosphopyridine nucleotide and diaphorase II, whilst the dehydro- 

 genation of malic acid to oxaloacetic acid requires the presence of 

 cozymase and diaphorase I. Nor is this all, for the tricarboxylic acid 

 cycle is now known to be intimately linked with fatty acid meta- 

 bolism through oxaloacetic acid, which is the pivot around which both 

 fat and carbohydrate metabolism revolve. It has been shown that 

 fatty acids with an even number of carbon atoms undergo j3-oxidation 

 to acetoacetic acid or a homologue, and that these are then converted 

 into acetyl phosphate which condenses with oxaloacetic acid to give 

 czs-aconitic acid. This is transformed into isocitric acid which then 

 passes through the tricarboxylic acid cycle. Propionic acid is oxidised 

 to pyruvic acid, which is either converted into acetyl phosphate or 

 metabolised through the tricarboxylic acid cycle. These changes are 

 represented as follows : 



CH3 . CH2 . CH2 . COOH 



butyric acid 



CH, . CHj . COOH 



propionic acid 



CH3 . CO . COOH 



pyruvic acid 



+ CO2 



CH3 . CO . CH2 . COOH 



acetoacetic 



CH3 . CO' 



acetyl phosphate 



HOOC . CH2 . CO . COOH 



oxaloacetic acid 



TRICARBOXYLIC 



ACID 



CYCLE 



CH . COO H 



C . COOH > 



CH2 . COOH 

 ci»-acoiiltic acid 



In the absence of enzymes containing nicotinic acid or ribofiavine 

 the metabolism of both fatty acids and carbohydrates is blocked, and 

 this explains why the presence of these two substances is vital for the 

 continued existence of all living cells. 



Ribofiavine and nicotinic acid are not the only B vitamins essential 

 for fatty acid and carbohydrate metabolism, however. Aneurine 

 pyrophosphate is a coenzyme necessary for the conversion of p5T:uvic 

 acid into lactic acid, acetic acid, acetoacetic acid and oxaloacetic 

 acid ; probably it activates the molecule preparatory to its oxidation 



627 



