Xanthurenic acid structure

A predominant toxin (51) from water beetles of the genus llybius (Table V) shows a UV absorption corresponding to hydroxyquinoline or hydroxyiso-quinoline. The H-NMR spectrum exhibits, beside signals of methyl ester and phenol, signals of five aromatic protons as both ABC and AB systems, the latter indicating two protons at C-3 and C-4 in quinoline. Since electron pyrolysis of 51 gives radioactive 8-hydroxyquinoline, its structure is identified as methyl 8-hydroxyquinoline-2-carboxylate (51) and confirmed by synthesis from xanthurenic acid (52) (Scheme 48) (101). The precursor of this alkaloid was shown to be tryptophan (444). 

Kyrmreninase (EC 3.7.1.3). Failure to convert 3-hy-droxykynurenine to 3-hydroxyanthranilic acid, and kynurenine to anthranilic acid. Increased urinary xanthurenic acid, kynurenine and 3-hydroxykynurenine, especially after ingestion of tryptophan. Some patients mentally retarded, others symptomless. Enzyme may not be absent, but structurally abnormal, with decreased affinity for coenzyme (dietary vitamin Bj temporarily corrects metabolic disturbance, and addition of pyridoxal phosphate to liver biopsy material increases enzyme activity to near normal). See Tryptophan. 

Xanthurenic acid was isolated from urine by Musajo in 1935 and shown to be 4,8-dihydroxyquinoline-2-carboxylic acid. Because of its 8-hydroxyquinoline structure it forms colored chelates with metal ions and, in particular, gives an intense green color with ferric salts. Xanthurenic acid is only excreted in pyridoxine deficiency, as was discovered by Lepkovsky and co-workers. It is found only in pyridoxine-defi-cient animals fed L-tryptophan or kynurenine. Addition of pyridoxine or the elimination of tryptophan from the diet causes it to disappear from the urine. ... 

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