COENZYMES DERIVED FROM B VITAMINS 141 



according to the following outline (in which some of the more important 

 specific systems have been included as examples) : 50, 51 



Aldehyde — primary alcohol (acetaldehyde — ethanol) 



Ketone — secondary alcohol (pyruvic acid — lactic acid) 



(oxalacetic acid — malic acid) 

 (oxalsuccinic acid — isocitric acid) 

 (/3-hydroxybutyric acid — acetoacetic 

 acid) 

 Acyl phosphate — aldehyde-1-phosphate (1,3-diphosphoglyceric acid — 1,3-diphos- 



phoglyceraldehyde) 

 Acid — -aldehyde (hydrate) (gluconic acid — glucose) 



( phosphogluconic acid — glucoses-phos- 

 phate) 

 ( phosphoglyceric acid — phosphoglycer- 

 aldehyde) 

 Imine — amine (iminoglutaric acid — L-glutamic acid) 



Both the glucose-6-phosphate and isocitric acid systems, as well as the 

 glutamic acid dehydrogenase of yeast and bacteria, 50 must be coupled 

 with the triphospho nucleotide; the glutamic dehydrogenase system of 

 animal tissue requirement is nonspecific; and all the other well charac- 

 terized enzyme systems react most readily with the diphospho coenzyme. 

 Several other reactions have been reported to be activated when these 

 coenzymes are added to specific substrates in the presence of unrefined 

 preparations of the apoenzymes. These include several interesting chemi- 

 cal transformations; the reduction of nitrate to nitrite, 52 the oxidation of 

 luciferin (the substrate that on oxidation produces bioluminescence) , 53 

 and the dehydrogenation of formic acid. 54 When crude enzyme prepara- 

 tions are used, there is always a chance of the observed reaction being a 

 secondary effect and further characterization should be undertaken before 

 the reaction is definitely catalogued as to its type. 



The Riboflavin-Containing Coenzymes 



The role of riboflavin-containing enzymes as hydrogen carriers was 

 first definitely established in 1932 when a yellow enzyme isolated from 

 yeast was found to serve as a connecting link in a complex system in 

 which hexose monophosphate is oxidized and molecular oxygen reduced. 55 

 The specific purpose of the jlavoprotein in this system is to catalyze the 

 reaction in which the hydrogen atoms accepted by triphosphopyridine 

 nucleotide from the .substrate are passed on to oxygen. Later, other flavo- 

 proteins were found to function in a comparable fashion with diphospho- 

 pyridine nucleotide, and it became evident that one of the fundamental 

 purposes of the riboflavin enzymes is to serve as "bridges" over which the 

 hydrogen atoms accumulating on the nicotinic acid coenzymes can be 

 passed to systems which will carry out the reduction of oxygen. An 



