CHEMOTHERAPEUTIC AGENTS I47 



*folic acid' requirements (citrovorum factor or folinic acid) 

 of Leuconostoc citrovorum are replaceable by a synthetic 

 material, A^-5-formyl-5, 6, 7, 8-tetrahydropteroylglutamic 

 acid, but not by pteroylglutamic acid itself (synthetic folic 

 acid). The experimental evidence favours the view that 

 organisms utilize PAB solely for the synthesis of 'folic acid' 

 but the detailed constitution of the natural coenzyme con- 

 taining combined PAB is still unknown [29]. 



The role of PAB in intermediary metabolism was ex- 

 plored further by a comprehensive survey of the substances 

 other than folic acid which were capable of antagonizing the 

 growth inhibitory action of the sulphonamides. This proce- 

 dure is based on the hypothesis that if PAB or a derivative 

 functions catalytically in the synthesis of substances essential 

 for growth, and if the sulphonamides act by preventing the 

 normal functioning of PAB, then growth should be resumed 

 if these substances are supplied exogenously. In a sense 

 such cultures can be regarded as being deficient in PAB 

 or folic acid, and the principles involved are the same as 

 in the growth factor replacement technique in which an 

 attempt is made to replace a grov^rth factor either by simpler 

 substances from which it can be synthesized, or by sub- 

 stances whose synthesis the growth factor is suspected to 

 mediate. Apart from folic acid, the natural antagonists of 

 the sulphonamides fall into three groups, (i) amino-acids, 

 in particular methionine and serine, (ii) purines such as 

 xanthine and (iii) thymine and thymidine [22, 27]. For 

 example, the addition of methionine to the medium 

 decreased the amount of PAB required to overcome the 

 inhibitory effects of sulphanilamide on the growth of Esch. 

 colt. The PAB requirement was further reduced if xanthine 

 was also included and diminished still further, if the 

 medium contained methionine, xanthine and serine. It was 

 abolished altogether when thymine was added in addition 

 to these three substances. Similarly with a PAB auxotroph 

 of Esch. coll growth was possible in the absence of PAB, 

 provided the medium contained methionine, a purine and 

 thymine. Analogous experiments have been done with 

 other organisms and the same three groups of substances 



