COMPETITIVE ANALOGUE-METABOLITE INHIBITION 475 



in the isolation of the factors from liver or in the assay of relatively 

 good sources of this group of factors. 5 



Whether the biosynthesis of methionine, purines, serine or thymine (or 

 folic acid) is the limiting reaction in the growth of Escherichia coli 

 inhibited by sulfanilamide, the inhibition index is increased about three- 

 fold by the addition of erythrotin at a concentration of 0.0005 y per 

 10 cc as indicated in Table l. 5 This effect is not enhanced by increasing 

 the concentrations of erythrotin, even to 0.1 y per 10 cc. From these 

 results it is apparent that erythrotin plays a catalytic role in the bio- 

 synthesis of methionine, purines, serine, thymine and additional products. 

 The catalytic role of erythrotin involves either the conversion of 

 p-aminobenzoic acid to the coenzyme form involved in these syntheses or 

 a separate catalytic role in combining the single carbon unit into these 

 metabolites. Thus, the close association of the functioning of the vitamin 

 B i2 group and p-aminobenzoic acid is further indicated. The biological 

 action of other unknown factors which are currently being isolated by 

 similar techniques will further clarify the exact role of this new vitamin. 



Table 1. Effect of Erythrotin on Sulfanilamide Inhibition of E. coli 



-Inhibition Index- 



With added 

 Without added Erythrotin 



Supplement Erythrotin 0.00057 per 10 cc 



None 3,000 10,000 



Methionine, 100 y per 10 cc 10,000 30,000 



Methionine, 100 y per 10 cc \ m nnn , nn nnn 



Xanthine, 100 y per 10 cc / 30 ' 000 100,000 



Methionine, 100 y per 10 cc "i 



Xanthine, 100 y per 10 cc J 50,000-100,000 200,000-300,000 



Serine, 100 y per 10 cc J 



Methionine, 100 y per 10 cc l 



Xanthine, 100 t per 10 cc I 100,000-200,000 300,000-500,000 



serine, 100 y per 10 cc ' ' ' ' 



Folic acid, 0.03 y per 10 cc > 



It is interesting that neither thymidine nor other desoxyribosides which 

 replace the vitamin Bi 2 group for both Lactobacillus lactis and Lacto- 

 bacillus leichmanni is active in replacing erythrotin for Escherichia 

 coli. 5 Similarly ascorbic acid, glutathione and related compounds at con- 

 centrations which can replace vitamin B i2 under certain conditions for 

 these lactic acid bacteria do not affect appreciably the response of 

 Escherichia coli in the sulfonamide assay. 5 



Mechanisms of Resistance to Competitive Inhibitors 



If an organism is allowed to grow in the presence of sub-inhibitory con- 

 centrations of a vitamin analogue or drug, the concentration of the in- 



