220 THE BIOCHEMISTRY OF B VITAMINS 



When this stage of the process is reached, the organism is confronted 

 with the problem of the disposition of the pyruvic acid and of the hydro- 

 gen atoms which have been temporarily taken up by the diphosphopyri- 

 dine nucleotide coenzyme. This coenzyme must be reconverted to the 

 oxidized form for re-use, for, if no suitable hydrogen acceptor can be 

 found, the glycolytic process will stop when all the coenzyme is tied up 

 in its reduced state. 



Under anaerobic conditions pyruvic acid itself or other products derived 

 from it acts as the acceptor for the hydrogen atoms. When oxygen is 

 available and can be utilized, it can serve as the final acceptor for the 

 hydrogen atoms and will in addition permit the pyruvic acid to be con- 

 verted, by reactions involving further dehydrogenations, into compounds 

 which are in a sense oxidation products; or pyruvic acid may be com- 

 pletely degraded to carbon dioxide and water. 



The reactions into which pyruvic acid can enter are numerous, and 

 we find a great deal of variation among the various forms of life in the 

 manner in which they carry forward the carbohydrate metabolism from 

 this point. 5 Probably most often several of the reactions are utilized 

 simultaneously, although one reaction may predominate to such an extent 

 that the others are completely overlooked. 



Anaerobic Utilization of Pyruvic Acid. When a cell has only a limited 

 supply of molecular oxygen or when it lacks the porphyrin enzymes 

 (cytochromes) which catalyze the utilization of molecular oxygen as 

 the final hydrogen acceptor, it must dispose of the hydrogen atoms 

 temporarily associated with the reduced nicotinic acid and of the pyruvic 

 acid by an anaerobic process. The anaerobic processes can be classified 

 in three groups. 



The simplest and most direct process is that in which the pyruvic acid 

 itself accepts the hydrogen atoms from the coenzyme forming lactic 

 acid. This type of reaction is the predominant method in a number of 

 bacteria and in the tissues of vertebrates. The only vitamin involved is 

 the nicotinic acid which is present in the reduced coenzyme. 



hexose unit 



DPN 



2 [2H] DPN 2 pyruvic acid 



DPN 



2 lactic acid 



A second important type process is one in which the reduction of 

 pyruvic acid takes place after it has undergone carboxylation. 9 Oxalacetic 



