Effects of dietary di(2-ethylhexyl)phthalate, a putative endocrine disrupter, on enzyme activities involved in the metabolism of tryptophan to niacin in rats.

We have reported that the conversion ratio of tryptophan to niacin increased with increasing dietary concentration of di(2-ethylhexyl)phthalate (DEHP); the conversion ratio was about 2.0% in the control rat, which increased by about 30% in the rat fed with 3.0% DEHP diet. In this study, we investigated whether this abnormal increase in the conversion ratio by DEHP occurred through the alteration of the enzyme activities involved in the metabolism of tryptophan to niacin. Rats were fed with a diet containing 0%, 0.1%, 0.5%, or 1.0% DEHP for 21 days. The nine kinds of enzyme activities involved in the biosynthesis and catabolism in the liver and kidney were measured. Based on previous findings that the formation of quinolinic acid and its' metabolites significantly increased with DEHP administration, we proposed that the activity of 2-amino-3-carboxymuconate-6-semialdehyde decarboxylase would be inhibited by DEHP intake. However, we found that the activities in the liver and kidney did not decrease in the rat fed with DEHP-containing diet. We discuss the discrepancy between the metabolite results and the enzyme activities.

Role of gustation in the recognition of oleate and triolein in anosmic rats.

Recent studies suggest a chemical perception of dietary fat in the oral cavity. To examine the role of gustation for the recognition of oleate and triolein, very short-term (5-min), two-bottle preference tests were conducted in anosmic rats. To minimize the effects of olfaction, texture and postingestive effects, rats were rendered anosmic with intranasal zinc sulfate, test substances were suspended in 0.3% xanthan gum solution and test fluids were offered for 5 min. Rats preferred oleate fluid but not triolein fluid to the control of 0.3% xanthan gum solution. The preference threshold for oleate in the rat oral cavity was between 0.2% and 0.5%. In the two-bottle preference tests between oleate and triolein, rats preferred oleate fluid to triolein fluid, showing discrimination of oleate and triolein. The results suggest that rat recognizes oleate by a gustatory cue and that fatty acid but not triglyceride is important for gustatory recognition of fat.

Growth-promoting activity of pyrazinoic acid, a putative active compound of antituberculosis drug pyrazinamide, in niacin-deficient rats through the inhibition of ACMSD activity.

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.

Effects of fatty liver induced by niacin-free diet with orotic acid on the metabolism of tryptophan to niacin in rats.

The effects of dietary orotic acid on the metabolism of tryptophan to niacin in weaning rats was investigated. The rats were fed with a niacin-free, 20% casein diet containing 0% (control diet) or 1% orotic acid diet (test diet) for 29 d. Retardation of growth, development of fatty liver, and enlargement of liver were observed in the test group in comparison with the control group. The concentrations of NAD and NADP in liver significantly decreased, while these in blood did not decrease compared to the control group. The formation of the upper metabolites of tryptophan to niacin such as anthranilic acid, kynurenic acid, and 3-hydroxyanthranilic acid were not affected, but the quinolinic acid and beyond, such as nicotinamide, N1-methylnicotinamide, N1-methyl-2-pyridone-5-carboxamide, and N1-methyl-4-pyridone-3-carboxamide, were significantly reduced by the administration of orotic acid. Therefore, the conversion ratio of tryptophan to niacin significantly decreased in the test group in comparison with the control group.

Elucidation of the toxic mechanism of the plasticizers, phthalic acid esters, putative endocrine disrupters: effects of dietary di(2-ethylhexyl)phthalate on the metabolism of tryptophan to niacin in rats.

We have previously reported that the administration of a large amount of di(n-butyl)phthalate (DBP) increased the conversion ratio of tryptophan to niacin in rats. In the present experiment, the effect of di(2-ethylhexyl)phthalate (DEHP) on the conversion ratio and how altering the conversion ratio of tryptophan to niacin depended on the concentration of DEHP were investigated to elucidate the toxic mechanism of phthalic acid esters (PhE). Rats were fed with a diet containing 0%, 0.01%, 0.05%, 0.1%, 0.5%, 1.0%, or 3.0% DEHP for 21 days. To assess the conversion ratio of tryptophan to niacin, urine samples were collected at the last day of the experiment and measured for metabolites on the tryptophan-niacin pathway. The conversion ratio increased with increasing dietary concentration of DEHP above 0.05%; the conversion ratio was about 2% in the control group, whereas it was 28% in the 3.0% DEHP group. It is suggested that the inhibition of alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase (ACMSD) by DEHP or its metabolites caused this increase in the conversion ratio. We conclude that PhE such as DEHP and DBP disturbed the tryptophan-niacin metabolism.

Identification of a toxic mechanism of the plasticizers, phtahlic acid esters, which are putative endocrine disrupters: time-dependent increase in quinolinic acid and its metabolites in rats fed di(2-ethylhexyl)phthalate.

We have reported that the administration of di(2-ethylhexyl)phthalate (DEHP) increased the formations of quinolinic acid (QA) and its lower metabolites on the tryptophan-niacin pathway. To discover the mechanism involved in disruption of the tryptophan-niacin pathway by DEHP, we assessed the daily urinary excretion of QA and its lower metabolites, and enzyme activities on the tryptophan-niacin pathway. Rats were fed with a niacin-free, 20% casein diet or the same diet supplemented with 0.1% DEHP or 0.043% phthalic acid and 0.067% 2-ethylhexanol added for 21 days. Feeding of DEHP increased the urinary excretions of QA and its lower metabolites in a time-dependent manner, and the increase of these excretions reached a peak at 11 days, but feeding of phthalic acid and 2-ethylhexanol had no effect. Feeding of DEHP, however, did not affect any enzyme activity including alpha-amino-beta-carboxymuconate-epsilon-semialdehyde decarboxylase (ACMSD), affecting the formation of QA, on the tryptophan-niacin pathway.