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ToxSci Advance Access originally published online on June 30, 2004
Toxicological Sciences 2004 81(2):302-308; doi:10.1093/toxsci/kfh204
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Toxicological Sciences vol. 81 no. 2 © Society of Toxicology 2004; all rights reserved.

Phthalate Esters Enhance Quinolinate Production by Inhibiting {alpha}-Amino-ß-Carboxymuconate-{varepsilon}-Semialdehyde Decarboxylase (ACMSD), a Key Enzyme of the Tryptophan Pathway

Tsutomu Fukuwatari*, Seiko Ohsaki*, Shin-ichi Fukuoka{dagger}, Ryuzo Sasaki* and Katsumi Shibata*,1

* Laboratory of Food Science and Nutrition, Department of Life Style Studies, School of Human Cultures, University of Shiga Prefecture, Hikone, Japan; and {dagger} Division of Food Science and Biotechnology, Graduate School of Agriculture, Kyoto University, Uji, Japan

Received March 18, 2004; accepted June 20, 2004

Tryptophan is metabolized to {alpha}-amino-ß-carboxymuconate-{varepsilon}-semialdehyde (ACMS) via 3-hydroxyanthranilate (3-HA). ACMS decarboxylase (ACMSD) directs ACMS to acetyl CoA; otherwise ACMS is non-enzymatically converted to quinolinate (QA), leading to the formation of NAD and its degradation products. Thus, ACMSD is a critical enzyme for tryptophan metabolism. Phthalate esters have been suspected of being environmental endocrine disrupters. Because of the structural similarity of phthalate esters with tryptophan metabolites, we examined the effects of phthalate esters on tryptophan metabolism. Phthalate esters containing diets were orally given to rats and the urinary excreted tryptophan metabolites were quantified. Of the phthalate esters with different side chains tested, di(2-ethylhexyl)phthalate (DEHP) and its metabolite, mono(2-ethylhexyl)phthalate (MEHP), most strongly enhanced the production of QA and degradation products of nicotinamide, while 3-HA was unchanged. This pattern of metabolic change led us to assume that these esters lowered ACMSD protein or its activity. Although DEHP could not be tested because of its low solubility, MEHP reversibly inhibited ACMSD from rat liver and mouse kidney, and also the recombinant human enzyme. Correlation between inhibition of ACMSD by phthalate esters with different side chains and urinary excretion of QA supports the notion that phthalate esters perturb tryptophan metabolism by inhibiting ACMSD. Quinolinate is a potential endogenous toxin and has been implicated in the pathogenesis of various disorders. Although toxicity of phthalate esters through accumulation of QA remains to be investigated, they may be detrimental by acting as metabolic disrupters when intake of a tryptophan-rich diet and exposure to phthalate esters occur coincidentally.

Key Words: phthalate ester; endocrine disrupter; tryptophan metabolism; quinolinate; metabolic disrupter.


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