148 NITROGEN METABOLISM 



are also active in antagonizing the growth inhibitory pro- 

 perties of analogues of folic acid, e.g. A:-methylfolic acid. 



Previous work with animal tissues and Neurospora indi- 

 cated that serine was synthesized by the addition of formate 



p-aminobenzoic acid 



HOOC 



precursors 



glycine + formate 



f 



serine 



precursors 



valine 



lysine 



threonine 



histidine 



homocysteine 

 methionine 



H© 



deoxyribosides 



of guanine, adenine 



and cytosine 



citrovorum 



factor 



(f clinic acid) 



combined PAB 

 Folic acid ? 



CHzCOOH 



ch.nh.oc-^~\nh .CH2-«| 



COOH 



pteroylglutamic acid 

 (synthetic folic acid) 



-t 



»""= 



precursors 



thymidine 

 thymine 



precursors 



amino- 

 imidazole 

 carboxamide 



MS) 



vitamin 

 ^2 



purines 



FIG. 10. 1. — Role of ^-aminobenzoic acid and vitamin B12 in 

 the synthesis of amino-acids, purines and pyrimidines 



to glycine, and since glycine, unlike serine, did not antagonize 

 sulphanilamide, it was concluded that the latter was an 

 inhibitor of this synthesis. Woods and his colleagues have 

 now shown that though Ln. mesenteroides is exacting towards 

 serine, growth occurred in the absence of the amino-acid 



