IV. BIOCHEMICAL SYSTEMS 139 



both jiroups of animals and thcMi administering PGA to one group for a 

 short time makes the experiment more specific for PGA and eliminates 

 general effects of inanition which might occur. Under these conditions, in- 

 corporation of formate into adenine and guanine in the hver is depressed 

 to approximately 30% of that in the PGA supplemented. Xo difference be- 

 tween deficient and repleted animals was noted in visceral purines. PGA 

 deficiency decreases the entrance of formate into the C-2 and C-8 positions 

 of liver purines equally, showing that PGA is involved in the incoiporation 

 of both these atoms. 



In the formation of 4-amino-5-imidazolecarboxamide during 4-amino- 

 pteroylglutamic acid inhibition of E. coli^^ the incorporation of the 2-carbon 

 is the only one affected. Apparently in this microorganism the incorporation 

 of the 8-carbon is not inhibited, as this is a part of the imidazole molecule. 

 If PGA is concerned equally with both C-2 and C-8, it is possible to under- 

 stand why the imidazole derivative has not been observed during growth 

 of any lactic acid organism on a medium devoid of PGA and supplemented 

 with purine. 



d. Thymine and Purines in Nutrition of Animals 



The abilit}'^ of th3''mine to replace PGA partially or completelj'' in the 

 nutrition of certain bacteria has prompted interest in whether thymine can 

 replace PGA in the nutrition of animals. In the rat (Daft^') and in the chick 

 (Stokstad e( al?) thymine is inactive both in growth stimulation and 

 hematopoietically. Thymine is ineffective for rats both on a PGA-free diet 

 containing sulfasuxidine and on a diet containing crude ''x-methyl PGA" 

 antagonist. Adenine is also ineffective in promoting growth in these experi- 

 ments. ]\Iore recently Daft^- has found that a combination of adenine and 

 thymine, each at a level of 1 %, produces a marked leucocyte and granulo- 

 C3'te response in rats rendered deficient in PGA by the feeding of sulfona- 

 mide. No growth response is oV)tained, however. The animals remain or be- 

 come anemic, and kidney damage develops. Adenine appears to account 

 for most of the activity of the mixture. It was further observed that the 

 agratnilocytosis .symptoms of PGA deficiency which had previously been 

 observed on low protein diets can be corrected by a mixture of methionine, 

 tryptophan, and threonine plus either adenine, adenosine, or yeast extract. 

 Previously, mixtures of the three amino acids had leen found inefTective 

 unless .<:ui)plemented with PGA. This suggests that PGA-deficient rats are 

 unable to synthesize amounts of adenine needed for white cell formation. 

 The apparent inactivity of thymine is in accord with the information gained 



5' F. S. Daft, Ann. N. Y. Acad. Set. 48, 299 (1946). 

 " F. S. Daft, Federation Proc. 11, 200 (1952). 



