Kynurenic Acid

KYNURENIC ACID (4-hydroxyquinoline-2-carboxylic acid), C₁₀H₇NO₃, forms crystals that are insoluble in cold water and relatively soluble in hot alcohol; it decomposes into CO₂ and kynurine (C₉H₇ON) at 287–288°. Its barium salt is characteristic, crystallizing in the form of colorless triangular plates. Kynurenic acid gives the Jaffe reaction; upon evaporation to dryness on a water bath with HCl and potassium chlorate, a reddish residue is formed, which turns first brownish-green and then emerald-green upon the addition of ammonia. Kynurenic acid has so far been found only in the urine of dogs and coyotes (the American prairie wolf).

Tryptophan serves as the material for the formation of kynurenic acid (Ellinger), with indole-pyruvic acid likely being an intermediate product. Tryptophan introduced into a dog or rabbit is partially converted into kynurenic acid. The content of kynurenic acid in dog urine increases with a meat diet and does not depend on the intensity of putrefactive processes in the intestine. Kynurenic acid is of interest in that it is almost the only representative of quinoline derivatives in the animal organism, whereas in plants, quinoline derivatives are encountered significantly more often (e.g., in various alkaloids). The process of the formation of kynurenic acid from tryptophan is also interesting, since the five-membered pyrrole ring of indole must be converted into a six-membered one.