Functional characterization and mechanism of action of recombinant human kynurenine 3-hydroxylase

NAD biosynthesis: identification of the tryptophan to quinolinate pathway in bacteria

Previous studies have demonstrated two different biosynthetic pathways to quinolinate, the universal de novo precursor to the pyridine ring of NAD. In prokaryotes, quinolinate is formed from aspartate and dihydroxyacetone phosphate; in eukaryotes, it is formed from tryptophan. It has been generally believed that the tryptophan to quinolinic acid biosynthetic pathway is unique to eukaryotes; however, this paper describes the use of comparative genome analysis to identify likely candidates for all five genes involved in the tryptophan to quinolinic acid pathway in several bacteria. Representative examples of each of these genes were overexpressed, and the predicted functions are confirmed in each case using unambiguous biochemical assays.

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.

Kidney and Liver Kynurenine Pathway Enzymes in Chronic Renal Failure

It has been suggested that kynurenine pathway may be an important pathological factor during the chronic renal failure (CRF) development. Therefore in the present study, in rats with end-stage of chronic renal failure, we measured the plasma and tissues (kidney and liver) concentrations of tryptophan, kynurenine, 3-hydroxykynurenine. We also evaluated the activity of tryptophan 2,3-dioxygenase in the liver (TDO), indoleamine 2,3-dioxygenase in kidney (IDO) and kynurenine 3-hydroxylase (HK) in the liver and in the kidney. The plasma and tissues tryptophan concentrations were decreased, whereas the concentrations of its metabolites increased when compared to control group. The increase in the TDO and 3-HK activity was observed, while IDO activity remains unchanged. In conclusion, the increase in the activity of TDO and HK along with disturbances of renal excreting function may be responsible for the elevation in the kynurenine and 3-hydroxykynurenine concentrations in experimental chronic renal failure.

cDNA cloning, functional expression and characterization of kynurenine 3-hydroxylase of Anopheles stephensi (Diptera: Culicidae)

Kynurenine 3-hydroxylase (K3H) is a NADPH-dependent flavin monooxygenase involved in the tryptophan pathway. Xanthurenic acid (XA) is a metabolite of this pathway and has recently been identified as a gamete activating factor (GAF) of the malarial parasite. We cloned K3H cDNA from Anopheles stephensi (AsK3H), because anopheline mosquitoes are a vector of the human malaria parasite, Plasmodium falciparum and the catalytic function of AsK3H in XA production. Recombinant AsK3H protein was expressed in Sf-9 cells using the baculovirus system and its enzymatic properties were characterized. The specific activities of crude cell lysate and affinity purified protein were 94.9 +/- 6.2 and 865.6 +/- 10.5 nmol/min/mg protein, respectively. The optimum pH of AsK3H was 7.0. Analysis of AsK3H gene expression using RT-PCR revealed that AsK3H was constitutively expressed in egg, larva, pupa and adult.