Kynurenine hydroxylase in Musca domestica L

Characterisation of white and yellow eye colour mutant strains of house cricket, Acheta domesticus

Two eye-colour mutant strains, white (W) and yellow (Y) of house cricket Acheta domesticus were established in our laboratory. We phenotyped and genotyped the mutants, performed genetic crossings and studied the eye structure and pigment composition using light and electron microscopy and biochemical analysis. We show that W and Y phenotypes are controlled by a single autosomal recessive allele, as both traits are metabolically independent. The analysis of the mutants`eye structure showed a reduced number of dark pigment granules while simultaneously, and an increased amount of light vacuoles in white eye mutants was observed. Significant differences in eye pigment composition between strains were also found. The Y mutant had a lower number of ommochromes, while the W mutant had a lower number of ommochromes and pteridines. This indicates that mutated genes are involved in two different, independent metabolic pathways regulating tryptophan metabolism enzymes, pigment transporter granules or pigment granule formation.

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.

Analysis of the wild-type and mutant genes encoding the enzyme kynurenine monooxygenase of the yellow fever mosquito, Aedes aegypti

Kynurenine 3-monooxygenase (KMO) catalyses the hydroxylation of kynurenine to 3-hydroxykynurenine. KMO has a key role in tryptophan catabolism and synthesis of ommochrome pigments in mosquitoes. The gene encoding this enzyme in the yellow fever mosquito, Aedes aegypti, is called kynurenine hydroxylase (kh) and a mutant allele that produces white eyes has been designated khw. A number of cDNA clones representative of wild-type and mutant genes were isolated. Sequence analyses of the wild-type and mutant cDNAs revealed a deletion of 162 nucleotides in the mutant gene near the 3'-end of the deduced coding region. RT-PCR analyses confirm the transcription of a truncated mRNA in the mutant strain. The in-frame deletion results in a loss of 54 amino acids, which disrupts a major alpha-helix and which probably accounts for the loss of activity of the enzyme. Recombinant Ae. aegypti KMO showed high substrate specificity for kynurenine with optimum activity at 40 degrees C and pH = 7.5. Kinetic parameters and inhibition of KMO activity by Cl- and pyridoxal-5-phosphate were determined.