Kynurenic Acid

Kynurenic acid (γ-hydroxyquinoline-carboxylic acid), C₁₀H₉NO₃, crystals, insoluble in cold water, fairly soluble in hot alcohol; decomposes into CO₂ and kynurine C₉H₇ON at 287-288°. The barium salt is characteristic, crystallizing in the form of colorless triangular plates. K. acid gives the Jaffe reaction; when evaporated to dryness on a water bath with HCl and potassium chlorate, it forms a reddish residue that turns first brown-green and then emerald-green with ammonia. K. acid has been found only in the urine of dogs and coyotes (American prairie wolf). Tryptophan (Ellinger) serves as the material for the formation of K. acid, with indole-pyruvic acid probably being an intermediate product. When administered to a dog or rabbit, tryptophan partially converts into K. acid. The content of K. acid in dog urine increases with a meat diet and does not depend on the intensity of putrefactive processes in the intestine. K. 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 much more frequently (e.g., in various alkaloids). The process of formation of K. acid from tryptophan is also interesting, as during this process the five-membered pyrrole ring of indole must transform into a six-membered ring.