72 THE BIOCHEMISTRY OF B VITAMINS 



tion, and in method 4, the organism has to be grown under anerobic con- 

 ditions. 



Assays for p-aminobenzoic acid are in general exacting partly because 

 laboratory equipment, glassware, etc., becomes so readily contaminated 

 with p-aminobenzoic acid that consistent assays are impossible. This is 

 particularly true when the more sensitive methods are used. Segregation 

 of the glassware used for this purpose and the use of a separate room are 

 desirable precautions, in addition to scrupulous care in the cleaning of 

 glassware. 



The problem of extraction of p-aminobenzoic acid, which has already 

 been discussed (p. 40) , is a crucial one and the four methods above have 

 never been compared under conditions where the extraction procedures 

 were the same. The values obtainable for different materials assayed are 

 therefore usually not comparable. The evidence indicates that the response 

 in every case is quite specific for p-aminobenzoic acid, in the sense that 

 no simple known chemicals have more than a slight effect. Information 

 is lacking as to how the unknown combined forms, which probably exist 

 naturally, affect the different organisms, and no tests involving pure folic 

 acid and its conjugates or rhizopterin have been found. 



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