IV. BIOCHEMICAL SYSTEMS 



125 



I I 



0=C2 4CH 



3 



N 

 H 



Thymine 



(5-methyluracil) 



O 



A 



7 ^^ 

 9 



HC2 4C— N^ 

 V/ H 



N 



Adenine 

 (6-Aminopurine) 



iCn 



reported that adenine or guanine stimulated the growth of Streptococcus 

 faccoUs l\ and that a further response was produced by thymine. Stokstad^ 

 found that the PCiA requirement of Laclohacilivs casei can be partially re- 

 placed by a combination of 0.5 7 of thymine and 5.0 7 of guanine per 

 milliliter of media. However, the maximum growth obtained by a combina- 

 tion of purine and pyrimidines is only about half that produced by concen- 

 trates of L. casei factor (P(iA). Thymine could not be replaced by other 

 pyrimidines such as uracil or cytosine. The purine requirements "were less 

 specific and could be met by guanine, xanthine, adenine, or hypoxanthine. 

 A number of pyrimidines, purines, and pterins have been tested by Stokes^ 

 as possible substitutes for thymine in the nutrition of S. faecalis R. Of 

 these, thymidine (thymine dosoxyriboside) is the only substance which has 

 thymine-like activity equal to that of thymine on a molar basis. Thymus 

 nucleic acid is inactive, which demonstrates the inability of S. faecalis R to 

 hydrolyze this nucleic acid to the riboside form. Xo information is available 

 on the activitj^ of thymidine desoxynucleotidc. A study of pyrimidine 

 analogs reveals that a methyl group in the 5 position is essential for ac- 

 tivity. Shifting the methyl group to the 1, 3 or 4 ])osition, or replacing it by 

 an ethj'l, amino, or nitro group, results in complete loss in activity. 



A possible role of thymine in serving as a substitute for PGA was sug- 

 gested on the basis of experiments with these two compounds in the nutri- 

 tion of S. faecalis R and L. casei} It was found that, whereas PCJA alone 

 would give a response with S. faecalis R, the maximum effect was not ob- 

 tained unless a purine also was added. In the presence of a purine five 

 thousand times as much thymine as PGA is needed for maximum growth. 

 With S. faecalis R, thymine gives the same maximum growth rate as can 

 1)0 obtained wilh ITIA, although with /.. casei only half-maximum growth 

 i^ achieved. 



The possibility of formation of PGA by ,S. faecalis \\ cells during growth 



\:. 1.. 1{. Stokstiul, ./. Biol. ('hem. 139, 475 (1!»41). 

 J. I.. Stokes, ./. Bacterial. 48. 201 (1944). 



