Xanthurenic index

Another line of research was initiated by Chiancone (C4a) in 1950, using a tryptophan loading test. The xanthurenic index of Chiancone expresses, as a percentage of the administered tryptophan, the amount of xanthurenic acid excreted within 24 hours after tryptophan ingestion. 

Xanthurenic and kynurenic acids, and kynurenine and hydroxykynurenine, are easy to measure in urine, so the tryptophan load test, the ability to metabolize a test dose of 2 to 5 g (150 to 380 /xmol per kg of body weight) of tryptophan, was widely adopted as a convenient and sensitive index of vitamin Bg nutritional status. 

Peripheral neuropathy has been observed as a complication in tuberculosis therapy with isonicotinic acid hydrazide (isoniazid), especially when large doses have been employed (B14). This complication of isoniazid therapy has been largely eliminated by simultaneous administration of pyridoxine. These observations have prompted studies on the urinary excretion of xanthurenic acid as an index of the antipyridoxine activity of isoniazid. The excretion of an excess of vitamin Be as such... 

Xanthurenic and kynurenic acids, and kynurenine and hydroxykynurenine, are easy to measure in urine, so the tryptophan load test (the ability to metabolize a test dose of 2—5 g of tryptophan) has been widely adopted as a convenient and very sensitive index of vitamin nutritional status. However, because glucocorticoid hormones increase tryptophan dioxygenase activity, abnormal results of the tryptophan load test must be regarded with caution, and cannot necessarily be interpreted as indicating vitamin B deficiency. Increased entry of tryptophan into the pathway will overwhelm the capacity of kynureninase, leading to increased formation of xanthurenic and kynurenic acids. Similarly, oestrogen metabolites inhibit kynureninase, leading to results that have been misinterpreted as vitamin B deficiency. 

The ability to metabolise a test dose of tryptophan has been widely adopted as a convenient and sensitive index of vitamin Bg nutritional status. However, induction of tryptophan dioxygenase by glucocorticoid hormones will result in a greater rate of formation of kynurenine and hydroxykynurenine than the capacity of kynureninase, and will thus lead to increased formation of kynurenic and xanthurenic acids—an effect similar to that seen in vitamin Bg deficiency. Such results may be erroneously interpreted as indicating vitamin Bg deficiency in a variety of subjects whose problem is increased glucocorticoid secretion as a result of stress or illness, not vitamin Bg deficiency. 

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