METABOLIC FUNCTIONS OF B VITAMINS 



221 



acid is first formed by /?-carboxylation of pyruvic acid and on subsequent 

 reduction yields malic acid. In biological systems, malic acid is usually 

 in equilibrium with its dehydration product, fumaric acid. Whether or 

 not biotin is generally required to mediate the /?-carboxylation step is 

 still an open question (p. 171), and it may be that this method of carbo- 

 hydrate utilization is another instance in which the nicotinic acid 

 coenzyme is the only vitamin coenzyme participating. (Further reduc- 

 tion of the fumaric acid, utilizing hydrogen atoms from other metabolic 

 reactions, can occur and results in the production of succinic acid. The 

 enzyme needed for this reaction, a fumaric reductase, might be expected 

 to be one of the riboflavin enzymes) (p. 150). 



hexose unit 

 DPN 



2 [2H] DP n 



2 pyruvic acid 



I CO, (BIOTIN COENZYME) 

 2 oxalacetic acid 

 — — J.DPN 



2 malic acid 

 |-H 2 

 2 fumaric acid 



The third type anaerobic process is one in which pyruvic acid is first 

 decarboxylated, and the resulting C 2 compounds or their condensation 

 products are then reduced by the hydrogen atoms of the nicotinic acid 

 coenzyme. The thiamine coenzyme is believed to be absolutely essential 

 for the production of every one of these fermentation products which are 

 formed from pyruvic acid by decarboxylation. 3 However, they may be 

 divided into two subgroups depending upon whether or not there is 

 a pantothenic acid requirement. This requirement appears to be directly 

 determined by the type of decarboxylation. Those processes in which 

 thiamine catalyzes a simple decarboxylation to produce acetaldehyde or 

 its dimer, acetylmethylcarbinol, do not require pantothenic acid. Con- 



hexose unit 

 DPN 



2 [2H]dpn 



2 pyruvic acid 



I THIAMINE PYROPHOSPHATE 

 2 acetaldehyde + 2 C0 2 



\ DPN | 



2 ethanol 2 C0 2 



