Biosynthesis of xanthurenic acid 8-O-beta-D-glucoside in Drosophila. Characterization of the xanthurenic acid:UDP-glucosyltransferase activity

Xanthurenic acid 8-glucoside is a side metabolite of the tryptophan-xanthommatin pathway in Drosophila. From 3-hydroxykynurenine, two biosynthetic pathways can be envisaged, one via xanthurenic acid, and another via 3-O-glucoside of 3-hydroxykynurenine. In this report evidence is presented to show that the synthesis takes place via xanthurenic acid. (a) We have demonstrated that the Drosophila melanogaster vermilion purple mutant (unable to synthesize 3-hydroxykynurenine) synthesizes xanthurenic acid 8-glucoside when fed with xanthurenic acid; and (b) the activities required for its synthesis via xanthurenic acid have been found (3-hydroxykynurenine transaminase and xanthurenic acid:UDP-glucosyltransferase). This is the first time that a UDP-glucosyltransferase activity that utilizes xanthurenic acid has been demonstrated. The enzyme in crude extracts from Drosophila sordidula shows the following characteristics. (a) It has optimal activity at 35 degrees C at pH 7.1 (in buffer Tris-HCl), and in the presence of a divalent cation (Mg2+ or Mn2+); (b) the activity is inhibited by xanthurenic acid (above 1.5 mM), UDP, D-gluconic acid 1,5-lactone, and Triton X-100; (c) it is localized in both the microsomal and the soluble fractions; (d) the specific activity is two times higher in heads than in bodies; and (e) the activity is enhanced in flies fed with phenobarbital.     

Effect of xanthurenic acid on infectivity of Plasmodium falciparum to Anopheles stephensi.

Terminally differentiated malarial gametocytes remain in the vertebrate circulation in a developmentally arrested state until they are taken up by the mosquito. The gametocytes then undergo gametogenesis in the mosquito mid-gut within minutes after ingestion of the infected blood meal. The male gametogenesis (exflagellation) can be triggered by the combination of a decrease in temperature of at least 5 degrees C and a simultaneous increase in pH between 8.0 and 8.3. Xanthurenic acid, which is present in mosquito mid-gut as well as in mosquito head, had been shown to induce exflagellation in vitro at a non-permissible pH. Here we report for the first time that with the increasing concentration of exogenous xanthurenic acid, there is a gradual increase in the number of oocysts in the mid-gut of infected mosquitoes. The concentration of xanthurenic acid for optimum infection in the membrane feeding assay was determined to be 100 microM. Three different strains of Plasmodium falciparum, viz. 3D7, 7G8 and W2 were tested in different experiments and similar findings hold true for all of them. These results demonstrate that xanthurenic acid not only induces exflagellation of male gametocytes but also promotes infectivity of Plasmodium falciparum to mosquito vectors.     

Distribution of xanthurenic acid glucoside in species of the genus Drosophila

Xanthurenic acid 8-O-β-d-glucoside is a side metabolite of the tryptophan-xanthommatin pathway in Drosophila melanogaster, that is accumulated in some eye-color mutants. Since this compound had only been found in this species, we have analyzed 29 other Drosophila species for the occurrence of this compound using cellulose thin-layer chromatography. Xanthurenic acid glucoside was detected in 10 of them (8 of them belong to the Sophophora subgenus). In these species xanthurenic acid glucoside, as well as other metabolites of the final steps of the dihydroxanthommatin pathway (3-hydroxykynurenine, xanthurenic acid and dihydroxanthommatin) were quantitated in order to gain a better understanding of the relationships of the metabolites of this pathway.     

Amides of xanthurenic acid as zinc-dependent inhibitors of Lp-PLA(2)

AX10185, the phenyl amide of xanthurenic acid, was found to be a sub-100nM inhibitor of Lp-PLA(2). However, in the presence of EDTA the inhibitory activity of AX10185 was extinguished while the enzymatic activity of Lp-PLA(2) did not change. Subsequent metal screening experiments determined the inhibition to be Zn(2+) dependent. Structure-activity relationship studies indicated the presence of the 4-hydroxy group to be critical and selected substituted phenyl, polycyclic, and cycloaliphatic amides of xanthurenic acid to be well tolerated.     

Urinary excretion ratio of xanthurenic acid/kynurenic acid as a functional biomarker of niacin nutritional status

The present study was conducted to survey functional biomarkers for evaluation of niacin nutritional status. Over 500 enzymes require niacin as a coenzyme. Of these, we chose the tryptophan degradation pathway. To create niacin-deficient animals, quinolinic acid phosphoribosyltransferase-knock out mice were used in the present study because wild type mice can synthesize nicotinamide from tryptophan. When the mice were made niacin-deficient, the urinary excretion of xanthurenic acid (XA) was extremely low compared with control mice; however, it increased according to the recovery of niacin nutritional status. The urinary excretion of kynurenic acid (KA) was the reverse of XA. Kynurenine 3-monooxygenase, which needs NADPH, was thought to be suppressed by niacin deficiency. Thus, we calculated the urinary excretion ratio of XA:KA as a functional biomarker of niacin nutrition. The ratio increased according to recovering niacin nutritional status. Low values equate with low niacin nutritional status.     

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

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.