FUNCTION 



Methionine also antagonised the inhibitory action of sulphanil- 

 amide on E. coli, having about one-third the activity of the purines 

 in this respect.^ It is possible therefore that ^-aminobenzoic acid 

 functions in the synthesis of methionine as well as of the purine bases, 

 although folic acid is apparently concerned only with the latter. 

 4-Amino-2-chlorobenzoic acid was a specific inhibitor of methionine 

 synthesis by E. coli. Methionine was also able to stimulate the growth 

 of an X-ray mutant of E. coli, giving a response additional to that 

 produced by ^-aminobenzoic acid.* Thymine and other purines were 

 inactive on this strain, and pteroylglutamic acid had less than o-ooi % 

 of the activity of _^-aminobenzoic acid, suggesting that the latter has 

 a function independent of its association with folic acid. 



On the other hand, the biological importance of ^-aminobenzoic 

 acid is undoubtedly associated in part with its conversion to folic 

 acid. Thus ^-aminobenzoylglutamic acid, pteroic acid, pteroyl- 

 glutamic acid and pteroyltriglutamic acid could replace ^-amino- 

 benzoic acid as a growth factor for L. arabinosus, although they were 

 all less active on a molar basis. ^ The inhibition of L. arabinosus by 

 sulphanilamide was antagonised non-competitively by these sub- 

 stances, with the exception of pteroic acid, as well as by thymine. 



References to Section 13 



1. E. E. Snell and H. K. IVIitchell, Arch. Biochem., 1942, 1, 93. 



2. R. D. Housewright and S. A. Koser, /. Infect. Dis., 1944. 75, 113 . 



3. W. Shive and E. C. Roberts, /. Biol. Chem., 1946, 162, 463. 



4. J. O. Lampen, R. R. Roepke and M. J. Jones, ibid., 1946, 164, 789; 



5. J. O. Lampen and M. J. Jones, ibid., 1947, 170, 133. 



563 



