COENZYMES DERIVED FROM B VITAMINS 151 



as yet no naturally occurring hydrogen donor has been found and the 

 catalytic role of the flavoprotein can be demonstrated only when an arti- 

 ficial donor (a leuco dye) is used. 



It is of interest to note that a flavoprotein can catalyze the conversion 

 of fumaric to succinic acid because it may be indicative of a general 

 function of flavoproteins which has not yet been established — that of 

 being a general catalyst for the production or saturation of ethylenic 

 bonds. A difficultly soluble protein, succinoxidase, catalyzes a similar 

 reaction (reverse direction) in which succinic acid is dehydrogenated and 

 a cytochrome is reduced. 91 Whether this enzyme is a flavoprotein is ques- 

 tionable, but it has been shown that the concentration of succinoxidase is 

 less in the tissues of animals depleted of riboflavin than in the tissues of 

 animals on adequate diets. 92 



The Cytochromes 



Numerous compounds which are derivatives of porphyrins are essential 

 for extremely diverse types of biological function ranging from the 

 oxygen-transporting duties of hemoglobin to the energy transformations 

 catalyzed by chlorophyll. The porphyrins of particular interest in a dis- 

 cussion of the functioning of the B vitamins are the ones which act in the 

 same oxidation systems as do riboflavin and nicotinic acid. These com- 

 pounds are the cytochromes. It is believed that in the cells of all aerobic 

 organisms the cytochromes act as the final mediators in most, but not all, 

 processes in which the utilization of oxygen takes place. 



The porphyrins have many properties in common with the B vitamins : 

 they are found in all forms of life which make use of aerobic processes; 

 their distribution parallels other B vitamins ; and they constitute a nutri- 

 tional requirement for some bacteria and protozoa. 93 Except for one class 

 of insects, 93a no higher types of life, however, have been encountered 

 which cannot synthesize their own requirements for the porphyrins. Since 

 all the known reactions of the porphyrin-containing enzymes are those 

 in which molecular oxygen is involved, it is not surprising that organisms 

 which are true anaerobes do not contain detectable amounts of porphyrin 

 derivatives. 



There is still considerable confusion regarding the identity and chemical 

 function of the cytochromes that have been recognized and characterized 

 almost entirely on the basis of their absorption spectra — cytochromes a, 

 ai, a 2 , a 3 , b and b 2 . 94 Two cytochrome components, cytochrome c and 

 cytochrome c oxidase, however, have been well characterized on the basis 

 of their enzymatic activity, and their function in biological oxidations 

 has been well established. 



