COENZYMES DERIVED FROM B VITAMINS 153 



metrically. 104 An even more sensitive method is a polarigraphic one. 105 

 The reoxidation of reduced cytochrome c is brought about by molecular 

 oxygen in the presence of cytochrome oxidase, a porphyrin-containing 

 protein so intimately bound within the structure of cells that it was once 

 assumed that it could not be extracted and obtained in solution. A soluble 

 preparation, however, has been recently reported. 106 The exact mechanism 

 of the reaction catalyzed by this enzyme is obscure. Since no hydrogen 

 peroxide results from the reaction, the oxygen molecule either is reduced 

 by a process which does not proceed through the reduction state corre- 

 sponding to hydrogen peroxide, or else this state constitutes an unstable 

 intermediate which is instantaneously decomposed. In view of the general 

 peroxidase activity of the iron-porphyrin enzymes, the latter postulate 

 seems the more reasonable. 



When the cytochrome-cytochrome c oxidase systems are blocked by 

 the heavy metal poisons, CN-, H 2 S, NaN 3 , etc., their function in recon- 

 structed systems or in intact cells can be partially taken over by appro- 

 priate autooxidizable dyes. When these artificial hydrogen carriers are 

 used, oxygen is always reduced to hydrogen peroxide. 



One more type of iron-porphyrin enzyme, catalase, 107 should be men- 

 tioned, to complete the presentation of the catalysts involved in dehydro- 

 genations. In all reactions in which the utilization of molecular oxygen 

 is accomplished by enzymes or catalysts not containing iron or copper, 

 hydrogen peroxide is formed. If the peroxide were not decomposed at once 

 it not only would destroy the enzyme system catalyzing its formation but 

 would inactivate most adjacent proteins as well. Consequently, catalase 

 is an essential component of all cells having aerobic metabolic processes, 

 and hence is found widely distributed in nature. 



Coenzymes Essential for the Carboxylation and Decarboxylation of Keto 

 Acids 



In the metabolic processes essential for life, a number of keto acids are 

 continually being formed and utilized. Probably no other type of com- 

 pound is capable of participating in such a variety of enzymatic reactions. 

 Because of this, they occupy key positions in most metabolic processes 

 and are the essential links which interconnect the metabolism of carbo- 

 hydrates, proteins, and fats. From the standpoint of molecular turnover, 

 one of the most important mechanisms by which these compounds are 

 metabolized is decarboxylation, resulting in the cleavage of a carbon-to- 

 carbon bond and formation of carbon dioxide (or sometimes its reductive 

 product, formic acid). The reactions can most conveniently be grouped 

 together as decarboxylase processes. On the basis of the coenzymes 

 needed as catalysts, this group can be broken down into two definite 

 classes: those which require thiamine pyrophosphate and those which do 



