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We have recently reported that the antituberculosis drug, pyrazinamide (PZA), caused a significant increase in the conversion ratio of tryptophan to niacin in rats. In the present work, we investigated whether or not pyrazinoic acid (POA), a putative metabolite of PZA, increased the conversion ratio of tryptophan to niacin. Weaning rats were fed with a niacin-free and tryptophan-limited diet (negative control diet), or with the negative control diet supplemented with 0.003% nicotinic acid (positive control diet) or 1% POA (test diet) for 27 days. The growth rate was almost same between the groups fed on the positive control diet and the test diet. Dietary POA significantly increased the conversion ratio of tryptophan to niacin. Although POA did not directly inhibit the activity of alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase (ACMSD), the rate-limiting enzyme in the tryptophan-niacin pathway, liver ACMSD activity was only not detected in the test diet group. These results suggest that a derivative of POA metabolized by rats inhibited the ACMSD activity.

To discover the role of the kidney in tryptophan degradation, especially tryptophan to niacin, rat kidneys were injured by feeding a diet containing a large amount of adenine. The kidney contains very high activity of aminocarboxymuconate-semialdehyde decarboxylase (ACMSD), which leads tryptophan into the glutaric acid pathway and then the TCA cycle, but not to the niacin pathway. On the other hand, kidneys contain significant activity of quinolinate phosphoribosyltransferase (QPRT), which leads tryptophan into the niacin pathway. The ACMSD activity in kidneys were significantly lower in the adenine group than in the control group, while the QPRT activity was almost the same, however, the formations of niacin and its compounds such as N1-methylnicotinamide and its pyridones did not increase, and therefore, the conversion ratio of tryptophan to niacin was lower in the adenine group than in the control group. The contents of NAD and NADP in liver, kidney, and blood were also lower in the adenine group. The decreased levels of niacin and the related compounds were consistent with the changes in the enzyme activities involved in the tryptophan-niacin metabolism in liver. It was concluded from these results that the conversion of tryptophan to niacin is due to only the liver enzymes and that the role of the kidney would be extremely low.

The effect of food restriction on the conversion ratio of tryptophan to niacin was investigated, because it is known that the conversion ratio is influenced by nutritional factors. A 20% casein diet was fed to rats ad libitum (control), 1/2 the food of the control. 1/4 the food of the control, or starved for 9 days, and urine samples were collected to measure the urinary excretion of such tryptophan metabolites as kynurenic acid, xanthurenic acid, and nicotinamide. The conversion ratio in the 1/2, 1/4, or starving group increased at day 1 of the experiment, but returned to the original value from day 2. Only in the starving group did the conversion ratio extremely increase from day 6 to day 9, being about 5-times higher than that of the original value on day 9. The possible mechanism by which the conversion ratio increased during food restriction is discussed.

Measurement of urinary xanthurenic acid (XA) has been used clinically to study a variety of disorders caused by vitamin B6 deficiency. To obviate some cumbersome steps of current methods for measuring XA in human urine, we have developed a simple fluorometric method. We apply the urine sample to a solid-phase extraction column containing trimethylaminopropyl group bound to silica, which enables us to purify and concentrate the XA from the urine without contamination from various tryptophan metabolites. The XA in the acidic eluate can then be quantified fluorometrically. The linearity of the proposed method extends from 0.2 to 10.0 mg/L. The method is precise, yielding day-to-day CVs for two pooled control specimens (1.08 and 1.90 mg/L) of 1.2% and 2.6%, respectively. Correlation studies with an established HPLC method and with a spectrophotometric procedure showed correlation coefficients of 0.99 and 0.98, respectively. Interference from vitamin C, uric acid, salicylate, acetaminophen, vanillylmandelic acid, and homovanillic acid was insignificant. The proposed method for urinary XA is rapid, simple, and suitable for routine use in the clinical laboratory.

To investigate how vitamin B6 (B6) deficiency affects the whole metabolism of tryptophan-niacin, rats were fed for 19 days with each of the following four kinds of diets; a complete 20% casein diet (control diet), the control diet without B6, the control diet without nicotinic acid, and the control diet without nicotinic acid and B6, and the urinary excretion of such tryptophan metabolites as kynurenic acid, xanthurenic acid, nicotinamide, N1-methylnicotinamide, N1-methyl-2-pyridone-5-carboxamide, and N1-methyl-4-pyridone-3-carboxamide each and the enzyme activities involved in tryptophan-niacin pathway were measured. The urinary excretion of kynurenic acid decreased while that of xanthurenic acid increased drastically in the two B6-deficient groups, when compared with the B6-containing groups. These results indicate that the rats fed with the B6-free diets were in the vitamin-deficient state. The conversion ratio was calculated from the ratio of the urinary excretion of sum of nicotinamide, N1-methylnicotinamide, N1-methyl-2-pyridone-5-carboxamide, and N1-methyl-4-pyridone-3-carboxamide, to the Trp intake. The ratio was statistically lower in the B6-free diet than in the B6-containing diet under the niacin-free conditions.

Female Wistar rats were given a single oral dose of aflatoxin B1, either alone or with a large amount of riboflavin. Biochemical and histologic studies were performed for 30 months. Nine animals of 19 in the aflatoxin-treated group and only 5 of 18 in the riboflavin-aflatoxin-treated group developed hepatomas. The number of rats was insufficient for tests of statistical significance to be fruitful. Urinary excretion of tryptophan metabolites was studied in aflatoxin- and riboflavin-treated rats after an oral administration of 10 mg tryptophan/100 g rat. Riboflavin did not affect the percentage of aflatoxin-treated animals with abnormal urinary excretion patterns, but did increase the magnitude of the disturbances in elimination of kynurenic and xanthurenic acids. The hepatic tryptophan-oxygenase activity was increased only in the two groups given riboflavin, and the levels of nucleic acids were the same in all groups.