Tryptophan load test

The microbiological assay is well suited for routine application. It is less time consuming and does not require the clinical supervision necessary in tryptophan-loading tests. 

Metabolic loading tests and the determination of enzyme saturation with cofactor measure the ability of an individual to meet his or her idiosyncratic requirements from a given intake, and, therefore, give a nearly absolute indication of nutritional status, without the need to refer to population reference ranges. A number of factors other than vitamin intake or adequacy can affect responses to metabolic loading tests. This is a particular problem with the tryptophan load test for vitamin Be nutritional status (Section 9.5.4) a number of drugs can have metabolic effects that resemble those seen in vitamin deficiency or depletion, whether or not they cause functional deficiency. 

Figure 9.4. Tryptophan load test for vitamin Be status. Tryptophan dioxygenase, EC 1.13.11.11 formylkynurenine formamidase, EC 3.5.1.9 kynurenine hydroxylase, EC 1.14.13.9 kynureninase, EC 3.7.1.3 kynurenine oxoglutarate aminotransferase, EC 2.6.1.7 andkynurenine glyoxylate aminotransferase, 2.6.1.63. Relative molecular masses (Mr) tryptophan, 204.2 kynurenine, 208.2 3-hydroxykynurenine, 223.2 kynurenic acid, 189.2 and xanthurenic acid, 205.2.

The tryptophan load test for vitamin Bg nutritional status (the ability to metabolize a test dose of tryptophan) is one of the oldest metabolic tests for functional vitamin nutritional status. It was developed as a result of observation of the excretion of an abnormal-colored compound, later identified as the tryptophan metabolite xanthurenic acid. 

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. 

It is apparent that abnormally increased excretion of kynurenine metabolites after a test dose of tryptophan cannot necessarily be regarded as evidence of vitamin Bg deficiency. This means that the tryptophan load test is unreliable as an index of status in epidemiological studies, although it is (probably) reliable in depletion/repletion studies to determine requirements. 

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. 

A preliminary investigation on tryptophan metabolism in aged subjects (over 70 years old) was carried out by Avogaro, Crepaldi, and Parpajola and by Benassi and Allegri, using the tryptophan loading test. 

When the tryptophan loading test was applied in 2 cases of cirrhosis of the liver, the results were the same as in normal persons, with a relatively slight urinary excretion of kynurenine and its acetyl derivative. 

Increased xanthurenic acid excretion after 10 g DL-tryptophan was demonstrated by Lerner et al. (L3) in 3 of 5 patients with rum fits. This metabolic defect was corrected by pyridoxine administration, as observed in a second tryptophan load test. Using the same xanthurenic acid test, significant vitamin Ba deficiency was not observed in patients with alcoholism and associated epilepsy, acute and chronic alcoholism, cirrhosis, acute hallucinosis-tremulousness, acute peripheral neuropathy, Wemicke-Korsakoff syndrome, and nonalcoholic, healthy individuals. It is postulated that pyridoxine deficiency is etiologically related to rum fits (L3). 

Knapp (K5) investigated the possibility of reduced vitamin Be supplies in people with skin diseases by using a 10-g DL-tryptophan loading test. Excessive xanthurenic acid excretion occurred in 18 of 43 cases, 8 with allergic dermatitis, 2 with eczema from exposure to light, 2 with exudative multiform erythema, and the remainder with various afihctions. 

Several tests are available for asses ing vitamin B status measurement of plasma levels of PLP, measurement of the percentage stimulation of red blood cell glula-matc-oxaloacetate aminotransferase, measurement of the daily excretion of urinary pyridoxic acid, and the tryptophan load test. 

At One time it was thought that women taking oral contraceptives were at risk for B deficiency. This notion seem-S to have been in error. The error was due to a misinterpretation of the tryptophan load lest. As mentioned earlier, a deficiency in vitamin B(,can induce the accumulation of specific intermediates of the tryptophan catabolic pathway and enhanced excretion in the urine. Oral contraceptives can also induce ar increase in the formation and excretion of specific intermediates by stimulating the activity of specific enzymes of the tryptophan catabolic pathway, This stimulation was responsible for the false indications of deficiency. Independently of the tryptophan load test, there continues to be some evidence for risk associated with the use of oral contraceptives. Oral contraceptive use may result in lowered levels of plasma vitamin Bf, Tlicsc lowered levels may result in a vitamin deficiency when coupled with pregnancy and lactation. 

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