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. 2007 Mar;53(3):254-63.
doi: 10.1016/j.jinsphys.2006.09.004. Epub 2006 Sep 17.

The tryptophan oxidation pathway in mosquitoes with emphasis on xanthurenic acid biosynthesis

Qian Han  1 , Brenda T Beerntsen, Jianyong Li
Affiliations

The tryptophan oxidation pathway in mosquitoes with emphasis on xanthurenic acid biosynthesis

Qian Han et al. J Insect Physiol. 2007 Mar.

Abstract

Oxidation of tryptophan to kynurenine and 3-hydroxykynurenine (3-HK) is the major catabolic pathway in mosquitoes. However, 3-HK is oxidized easily under physiological conditions, resulting in the production of reactive radical species. To overcome this problem, mosquitoes have developed an efficient mechanism to prevent 3-HK from accumulating by converting this chemically reactive compound to the chemically stable xanthurenic acid. Interestingly, 3-HK is a precursor for the production of compound eye pigments during the pupal and early adult stages; consequently, mosquitoes need to preserve and transport 3-HK for compound eye pigmentation in pupae and adults. This review summarizes the tryptophan oxidation pathway, compares and contrasts the mosquito tryptophan oxidation pathway with other model species, and discusses possible driving forces leading to the functional adaptation and evolution of enzymes involved in the mosquito tryptophan oxidation pathway.

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Figures

Fig. 1
Fig. 1
Biochemical pathway of tryptophan to xanthurenic acid in mosquitoes. TDO, tryptophan dioxygenase; KFM, kynurenine formamidase; KMO, kynurenine monooxygenase; KAT, kynurenine aminotransferase; HKT, 3-hydroxykynurenine transaminase.
Fig. 2
Fig. 2
Spectral characteristics and biochemical activity of the recombinant Ae. aegypti tryptophan 2,3-dioxygenase (TDO). (A) Spectral characteristics of the purified recombinant Ae. aegypti TDO (about 1 mg in 1.0 ml of phosphate buffer, pH 7.5). TDO is a heme-containing enzyme. The presence of an absorbance peak with a λmax at 405 nm indicates that the heme prosthetic group is present in the enzyme. (B) Production of formylkynurenine in a TDO and tryptophan reaction mixture. Presence of an initial lag phase in formylkynurenine production in a TDO and tryptophan reaction mixture suggests that Ae. aegypti TDO is present as an inactive form, and addition of tryptophan results in its progressive activation. Curve 1 and curve 2 illustrate absorbance change at 321 nm in tryptophan solutions in the absence and presence of TDO (20 μg/ml reaction mixture), respectively.
Fig. 3
Fig. 3
A phylogenetic tree of alanine glyoxalate aminotransferases (AGT) from several representative model species. The phylogenetic tree was generated using CLUSTALW program in biology workbench.
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