Xanthurenic Acid
Overview
Xanthurenic acid is a tryptophan metabolite produced via the kynurenine pathway, specifically reflecting pyridoxal 5'-phosphate (vitamin B6)-dependent kynureninase activity. It serves as a biomarker of vitamin B6 nutritional status and functional deficiency, as B6 is required for its conversion to 3-hydroxykynurenine. Elevated urinary levels after a tryptophan load indicate B6 deficiency, while normal levels suggest adequate status; high baseline levels may signal liver disease or pyridoxine-dependent disorders. Clinically, it is useful for assessing vitamin B6 status in at-risk populations, diagnosing pyridoxine-responsive conditions, and monitoring nutritional adequacy.
Clinical Use Cases
- Assessment of vitamin B6 deficiency in malnutrition, malabsorption, or chronic disease.
- Diagnosis of pyridoxine-dependent epilepsy or sideroblastic anemia.
- Evaluation of isoniazid-induced B6 deficiency in tuberculosis treatment.
- Monitoring response to vitamin B6 supplementation.
- Investigation of tryptophan metabolism disorders.
Specimen Types
- Urine (24-hour or post-tryptophan load).
- Plasma (for baseline levels).
Measurement Methods
- High-performance liquid chromatography (HPLC) with fluorescence or UV detection.
- Liquid chromatography-mass spectrometry (LC-MS/MS).
- Colorimetric assays after tryptophan loading.
- Gas chromatography-mass spectrometry (GC-MS).
Test Preparation and Influencing Factors
- Tryptophan loading test: 2-5 g oral L-tryptophan, collect urine 4-24 hours later.
- Vitamin B6 supplements or foods elevate conversion and lower xanthurenic acid.
- Isoniazid, hydralazine, or oral contraceptives induce functional B6 deficiency.
- Liver disease impairs kynurenine pathway metabolism.
- Renal function affects urinary excretion.
- Pregnancy increases B6 requirements and excretion.
Synonyms
- Xanthurenate.
- 4-(2-Amino-3-hydroxyphenyl)-2,4-dioxobutanoic acid.