COMPETITIVE ANALOGUE-METABOLITE INHIBITION 471 



requirement under these conditions include adenosine, xanthine, guanine 

 and inosine. Adenine not only is ineffective, but is usually synergistic 

 with sulfonamides in preventing growth of Escherichia coli under these 

 conditions. 37 Thus, the first limiting enzymatic reaction involves the 

 biosynthesis of methionine, probably from homocysteine, since the latter, 

 a precursor, does not affect the inhibition. 5, 3S The next limiting transfor- 

 mation, which becomes apparent when exogenous methionine is supplied 

 to the organism, is the biosynthesis of purines, presumably from 5(4)- 

 amino-4(5)-imidazolecarboxamide or a derivative of this amine, since 

 the amine accumulates in the medium under these conditions of sul- 

 fonamide inhibition. 39, 40 Although the amine stimulates growth of 

 Lactobacillus arabinosus in a manner similar to purines and disappears 

 slowly from the medium during the process, 5 many organisms including 

 Escherichia coli cannot utilize the amine, and it remains unchanged in 

 the medium. It appears likely that the free amine is derived from a con- 

 jugated form, e.g., riboside or desoxyriboside, which is the normal pre- 

 cursor of purines. 



Glycine, which increases the production of the aminoimidazolecarboxa- 

 mide in Escherichia coli, is apparently a precursor of the amine. 41 Al- 

 though less effective than glycine, threonine similarly increases the pro- 

 duction of the amine and is apparently converted into glycine by the 

 organism. The inability of serine to replace glycine, particularly in view 

 of reported mutants of Escherichia coli requiring either serine or glycine 

 for growth, indicates that the conversion of serine to glycine is blocked 

 by sulfanilamide. 41 



The involvement of p-aminobenzoic acid in the biosynthesis of serine 

 is further indicated by the effect of serine on the inhibition of growth of 

 Escherichia coli by sulfanilamide in a medium containing both methionine 

 and purines. 38 The inhibition index is increased from 30,000 to 50,000 — 

 100,000 by supplements of serine under these conditions. 5 - 3S 



Thymine and folic acid are somewhat interchangeable in affecting the 

 inhibition index determined in a medium containing methionine, purines 

 and serine. 38 The inhibition index is usually increased two- or threefold 

 to approximately 200,000 to 300,000. 5, 3S High concentrations of sul- 

 fanilamide which are necessary for determination of these high inhibition 

 indices often do not affect solely enzymes utilizing p-aminobenzoic acid; 

 consequently, the results are sometimes variable. 5 ' 38 



For each of these products to exert its effect on the inhibition of growth 

 of Escherichia coli by sulfanilamide, the products must be added in a 

 definite sequence as indicated in Figure 6. All products whose biosyntheses 

 are related to lower inhibition indices must be available to the organism 

 from an exogenous supply in order that a product whose biosynthesis is 



