METABOLISM OF THE B VITAMINS 355 



In tracer studies with DL-tryptophan-/3-C 14 , the labelled atom may be 

 found in kynurenic acid, but not in niacin. 



-CH 2 CH(NH 2 )COOH r^N-CO— CH 2 CH(NH 2 )COOH 



"* L J-NH 2 

 H 



This indicates clearly that the conversion does not involve the pyridine 

 ring of kynurenic acid as a ring precursor of niacin. 100 When, however, 

 DL-tryptophan-3-C 14 is employed, the tracer atom is found in the car- 

 boxylic acid group of niacin, 107 strongly suggesting that it is the benzene 

 ring of tryptophan which eventually becomes the pyridine ring of nicotinic 

 acid. 



CH 2 CH(NH 2 )COOH ^ VCOCH 2 CH(NH 2 )COOH 



V ^^-NH 2 



OH 



-COOH f VCOOH 



-NH 2 ~^ l^ N 



J 



The ring rearrangement presumably involved in the final step of the 

 process is of a type not entirely unknown to organic chemistry. Very 

 recently Henderson and co-workers 107a - c have shown that a probable 

 intermediate in this step is quinolinic acid. Rats on a 9% casein diet 

 containing little niacin but 2.5% added DL-tryptophan excrete 6-11. 

 mg of this acid per clay. The injection of 1. mM of tryptophan or 3- 

 hydroxyanthranilic acid into such rats increases their niacin excretion 

 tenfold, their F 2 excretion twenty-five to one hundredfold, and their 

 quinolinic acid excretion one hundred to three hundredfold. Quinolinic 

 acid is slightly active for rats in replacing dietary niacin, is active in 

 some Neurospora strains, and accumulates in the culture medium of 

 other strains. Rat liver slices or homogenates are able to convert 3- 

 hydroxyanthranilic acid to quinolinic acid, and quinolinic acid is con- 

 verted by weak acid to a substance having niacin activity for Lactobacil- 

 lus arabinosus. 



