BIOCHEMICAL REACTIONS AND THEIR CATALYSTS 113 



B 6 activity of pyridoxine for an organism is a measure of that organism's 

 enzymatic capacity for carrying out the preliminary oxidation (p. 179). 



For some cells the coenzyme, or compounds intermediate between the 

 coenzyme and vitamin in their chemical complexity, must be enzymati- 

 cally cleaved before absorption can take place, thus necessitating the 

 complete resynthesis within the cell. An interesting example is cited in a 

 report in which it was shown that if the monophosphate of thiamine is 

 supplied as the substrate for the formation of the coenzyme (which is a 

 di(pyro) -phosphate), the monophosphate must first be hydrolyzed to the 

 unphosphorylated thiamine. 10 In such cases the coenzyme or intermediate 

 will be useless nutritionally to an organism if it cannot elaborate the 

 extracellular enzymes needed to degrade the more complex forms of the 

 vitamin. It is for this reason that the coenzyme of thiamine cannot be 

 used to supply the nutritional requirements of some thiamine-requiring 

 yeasts. 11 



The chemical mechanisms involved in the biosynthesis of a coenzyme 

 may not always include a reaction involving the vitamin itself. Thus 

 some bacteria can convert ^-alanine into a pantothenic acid complex 

 more rapidly than they can produce the complex if pantothenic acid is 

 the initial substrate. 12 



Rate of Coenzyme Synthesis. How rapidly is a coenzyme synthesized 

 from a vitamin? Such conversions can be carried out very quickly by 

 most cells. This is demonstrated by the rapidity with which one can get 

 a response when a cell deficient in some coenzyme is supplied the essential 

 vitamin. There is, to be sure, some lag in time between the addition of the 

 vitamin to the culture medium and a detectable response due to the 

 coenzyme formed. It must be remembered that a deficient cell is essen- 

 tially a dormant cell, since the retardation of vital metabolic processes 

 forces most of the cell's activities to come to a standstill. It is only natural 

 that some time should elapse before the cell can reach an active state 

 again. In cells in which the deficiency is not acute the conversion of a 

 vitamin to a coenzyme may take place almost immediately. This is 

 demonstrated in the fermentation assay method for thiamine. 13 The yeast 

 used contains sufficient thiamine coenzyme to carry on some fermentation, 

 and the cells are in a moderately active phase. The addition of thiamine 

 to a medium containing such cells results in an immediate increase in the 

 rate of fermentation. 



Extent of Coenzyme Synthesis. There is insufficient information on 

 which one could base a general statement concerning the relative con- 

 centrations of all the vitamins and their coenzymes within cells. It ap- 

 pears, however, that intracellularly the vitamins are predominantly in the 

 form of their active coenzymes. In blood, for example, it has been shown 



