472 THE BIOCHEMISTRY OF B VITAMINS 



related to a higher inhibition index may exert its effect on the inhibition. 

 However, the order in which the products exert their effect may vary to 

 some extent under different testing conditions and with inocula treated 

 differently. For example, it is occasionally possible in Escherichia coli to 

 obtain an effect with folic acid in the absence of serine. 



The requirement of certain mutant strains of Escherichia coli for 

 p-aminobenzoic acid can be replaced by a mixture of amino acids, purines 

 and thymine, indicating further the involvement of p-aminobenzoic acid 

 in the biosynthesis of these substances. 42 



The index for inhibition of growth of Lactobacillus arabinosus by 

 sulfanilamide is increased from 100 to 1000 by purines. 37 For this organism 

 and Streptobacterium plantarum 10S, thymine at high concentrations in 

 the presence of purines is reported to prevent the toxicity of sulfonamides 

 over a wide range of concentrations. 43 Both purines and thymine are 

 necessary to replace the requirement of p-aminobenzoic acid for growth 

 of Lactobacillus arabinosus. 43 Thymine exerts a similar effect on the 

 toxicity of sulfanilamides for certain strains of Streptococcus jaecalis 

 and Streptococcus zymogenes. 44 



One common feature in the metabolic reactions in which p-aminobenzoic 

 acid participates is the involvement of a single carbon unit in the final 

 product. Hence, it appears that p-aminobenzoic acid functions directly or 

 indirectly in the introduction of single carbon units into purines, pyrimi- 

 dines, serine (from glycine), and methionine (from homocysteine or a 

 related precursor) . A close association of all these single carbon units with 

 the possibility of a common precursor is suggested by these results. The 

 incorporation of isotopically labelled formate in the 2 and 8 positions of 

 uric acid previously indicated a single carbon unit precursor for the 

 purines. 45 The involvement of a common precursor was further indicated 

 recently by results indicating the incorporation of isotopically labelled 

 formate into the /3-carbon atom of serine, 46 and the incorporation of 

 isotopically labelled methyl groups of choline into the /3-carbon atom of 

 serine. 47 



On the basis of published research the interrelationship of p-amino- 

 benzoic acid and folic acid cannot be completely described. Folic acid 

 has little or no activity in replacing the nutritional requirement of a 

 number of organisms for p-aminobenzoic acid. However, sulfonamides 

 prevent the biosynthesis of a microbiologically active form(s) of folic 

 acid in a wide variety of organisms which do not require folic acid for 

 growth (p. 490). Furthermore, the biological functions of folic acid closely 

 parallel those of p-aminobenzoic acid. Thus, thymine and purines replace 

 or partially replace the folic acid requirement of certain organisms, 48-50 

 and with Lactobacillus casei, inhibition analysis indicates that folic acid 



