114 THE BIOCHEMISTRY OF B VITAMINS 



that, whereas the thiamine, nicotinic acid, riboflavin, and pantothenic 

 acid occur as such in the serum, they are almost entirely in the form of 

 their respective coenzymes inside the erythrocytes. 14, 15> 16 > 17 It may be 

 that part or all of the conversion of a vitamin to a coenzyme takes place 

 during its absorption into the cell. Many absorptive processes are known 

 to involve phosphorylation. Intestinal mucosa, a tissue in which absorp- 

 tive mechanisms are extremely highly developed, has been shown to be 

 a good source for obtaining preparations which will carry out the en- 

 zymatic syntheses (phosphorylation) of thiamine and riboflavin co- 

 enzymes. 18, 19 



Do all cells possess the enzymes necessary to carry out the syntheses 

 of their coenzymes from the vitamins? The known cases in which a 

 vitamin must be furnished in the form of the intact coenzyme are rare. 

 Two related species of bacteria cannot utilize nicotinic acid or nicotin- 

 amide (p. 136). Their nutritional requirements are usually supplied in 

 the form of the intact coenzyme, but actually only a portion of the whole 

 coenzyme is necessary. Some bacteria do not respond to pantothenic acid, 

 but must have either the coenzyme or some substance of intermediate 

 complexity that has been elaborated from pantothenic acid. 12 In neither 

 of these cases can we say that the organisms have a specific requirement 

 for the coenzyme. Recently some organisms have been found which can- 

 not utilize unphosphorylated derivatives of pyridoxal. 20 The rarity of 

 cases to date in which the intact coenzymes have been shown to be essen- 

 tial nutritional factors might lead one to expect that they will seldom 

 be found to be irreplaceable nutrilites. To draw such a conclusion now 

 is unwise. The coenzymes of only half of the known vitamins are as yet 

 chemically identified, and suitable preparations of several of the identified 

 compounds have not been generally available. At present it is possible 

 to obtain from commercial sources only two coenzymes — cocarboxylase 

 (thiamine) and coenzyme I (nicotinic acid). Furthermore, a large number 

 of organisms have never been successfully grown on media whose chemical 

 composition is known. Many of the chemically undefined nutritive re- 

 quirements which can now be furnished only by natural extracts may be 

 found to be unidentified coenzymes (or their intermediates) of the known 

 B vitamins. 



Number of Coenzymes. Is more than one coenzyme synthesized from 

 each vitamin? Those coenzymes which function as carriers of specific 

 groups or atoms naturally will exist in two states. Hydrogen carriers will 

 exist in the oxidized and reduced forms. A coenzyme which transports an 

 active group (the active acetyl radical, for example) probably exists part 

 of the time in chemical combination with the active substance. The situ- 

 ation in which pyridoxamine phosphate may be formed from pyridoxal 



