VI. BIOGENESIS 



527 



of nicotinic acid metabolites and would promote growth. Knox and 

 Mehler^''' '^^ have shown that the liver of several animal species possesses 

 an enzyme system which converts tryptophan to kyniirenine through the 

 intermediate w-formylkynurenine. However, the latter compound also was 

 inactive in nicotinic acid-deficient rats." KrehP*^ and Bonner and Yanofsky^^ 

 have suggested that the well-known instability of kynurenine might ac- 

 count for these conflicting results. 



J 



CH,-CII-COOH 



Tryptophan 



O 



II 

 -C-CH.-CII-COOH 



I 



Kynurenine 



f^~V-C-CHj-CH-COOH 



OH 



3-Hydro.\y kynurenine 



rr 



COOH 



OHC y 



NH, 



OH 



3-Hydroxyantliranilic acid 



r^^^^COOH 



OHC i-COOH 

 NH, 



r^^^pCOOH 



Nicotinic acid 



^N 



COOH 

 COOH 



Quinolinic acid 



Fig. 10. Summary of known and probable steps in the conversion of tryptophan 

 to nicotinic acid. 



The situation with 3-hydroxyanthranilic acid is much clearer, since it 

 does support growth in nicotinic acid-deficient rats^^' ^a, 73, ^^2^Yi^L increases 

 the urinary excretion of nicotinic acid metabolites.^** Schweigert^^ reported 

 that rat liver slices would convert 3-hydroxyanthranilic acid to a compound 



'^ W. E. Knox and A. H. Mehler, J. Biol. Chem. 187, 419 (1950). 



" A. H. Mehler and W. E. Kno.x, J. Biol. Chem. 187, 431 (1950). 



'^W. A. Krehl, Unpublished data quoted in Vitamins and Hormones 7, 140 (1949). 



" H. K. Mitchell, J. F. Nye, and R. 1). Owen, J. Biol. Chem. 175, 433 (1948). 



'8 P. W. Albert, B. T. Scheer, H. J. Deuel, Jr., J. Biol. Chem. 175, 479 (1948). 



^9 B. S. Schweigert, J. Biol. Chem. 178, 707 (1949). 



