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6-Hydroxykynurenic acid is a member of quinolines. 6-Hydroxykynurenic acid is a natural product found in Ephedra pachyclada, Ephedra regeliana, and other organisms with data available.

4,6-Dihydroxy-2-quinolinecarboxylic acid is found in fats and oils. 4,6-Dihydroxy-2-quinolinecarboxylic acid is an alkaloid from Ginkgo biloba (ginkgo).

Xanthurenic acid, also known as xanthurenate or 8-hydroxykynurenic acid, is a member of the class of compounds known as quinoline carboxylic acids. Quinoline carboxylic acids are quinolines in which the quinoline ring system is substituted by a carboxyl group at one or more positions. Xanthurenic acid is slightly soluble (in water). Xanthurenic acid can be found primarily in blood, feces, and urine, as well as in human epidermis tissue. Within the cell, xanthurenic acid is primarily located in the membrane. Xanthurenic acid exists in all eukaryotes, ranging from yeast to humans. In humans, xanthurenic acid is involved in the tryptophan metabolism. Moreover, xanthurenic acid is found to be associated with citrullinemia type I, which is an inborn error of metabolism. Xanthurenic acid is a metabolite from tryptophan catabolism. It is a substrate of the enzyme methyltransferases (EC 2.1.1.-) in pathway tryptophan metabolism (KEGG).

Zeanic acid is found in cereals and cereal products. Zeanic acid is isolated from corn steep liquor.

3-Hydroxykynurenic acid is a member of quinolines. 3-Hydroxykynurenic acid is a natural product found in Aplysina cavernicola with data available.

A quinolone that is 4-quinolone substituted at positions 2 and 3 by hydroxy and carboxy groups respectively." []