Reduced kynurenine aminotransferase-I activity in SHR rats may be due to lack of KAT-Ib activity

Perivascular brown adipocytes-derived kynurenic acid relaxes blood vessel via endothelium PI3K-Akt-eNOS pathway

OBJECTIVE

Several metabolites from the kynurenine pathway of tryptophan metabolism play a critical role in vascular function and vascular wall remodeling. This study aimed to test whether metabolite kynurenic acid (KYNA) from the kynurenine pathway relaxes blood vessels.

APPROACH AND RESULTS

We employed histological staining, in vitro cell culture, Western blotting, real-time PCR, and nitric oxide detection to validate kynurenine aminotransferase (KAT) localization in the vasculature as well as KYNA action on endothelial cells. We also detected vascular reactivity by organ chamber and monitored blood pressure by telemetry to investigate the regulation effect of KYNA on vascular tone. The results presented that perivascular adipose tissue (PVAT) from mice thoracic aorta had robust staining of anti-KAT1 and KYNA than PVAT from the abdominal aorta and mesenteric artery, which is consistent with the expression profile of brown adipocyte marker uncoupling protein 1. KYNA, metabolized from kynurenine by KAT, relaxed pre-contracted both aortic ring and mesenteric artery. In addition, KYNA derived from KAT in PVAT participates in the cross-talk between PVAT and vessel by mediating PVAT inhibition on agonist-induced thoracic aorta contraction. Furthermore, intraperitoneal injection of KYNA in mice reduced blood pressure. The vessel relaxation effect of KYNA was through the endothelium-dependent PI3K-Akt-eNOS pathway. Finally, the high-fat diet decreased KAT1 expression in perithoracic aortic fat and led to KYNA reduction in blood.

CONCLUSIONS

Our research identified KYNA generated by KAT as a novel perivascular brown adipocyte-derived vascular relaxation factor and suggests that KYNA reduction is a critical event in vascular dysfunction under obese condition.

pH dependence, substrate specificity and inhibition of human kynurenine aminotransferase I

Human kynurenine aminotransferase I/glutamine transaminase K (hKAT-I) is an important multifunctional enzyme. This study systematically studies the substrates of hKAT-I and reassesses the effects of pH, Tris, amino acids and alpha-keto acids on the activity of the enzyme. The experiments were comprised of functional expression of the hKAT-I in an insect cell/baculovirus expression system, purification of its recombinant protein, and functional characterization of the purified enzyme. This study demonstrates that hKAT-I can catalyze kynurenine to kynurenic acid under physiological pH conditions, indicates indo-3-pyruvate and cysteine as efficient inhibitors for hKAT-I, and also provides biochemical information about the substrate specificity and cosubstrate inhibition of the enzyme. hKAT-I is inhibited by Tris under physiological pH conditions, which explains why it has been concluded that the enzyme could not efficiently catalyze kynurenine transamination. Our findings provide a biochemical basis towards understanding the overall physiological role of hKAT-I in vivo and insight into controlling the levels of endogenous kynurenic acid through modulation of the enzyme in the human brain.

Crystal structures of Aedes aegypti kynurenine aminotransferase

Aedes aegypti kynurenine aminotransferase (AeKAT) catalyzes the irreversible transamination of kynurenine to kynurenic acid, the natural antagonist of NMDA and 7-nicotinic acetycholine receptors. Here, we report the crystal structure of AeKAT in its PMP and PLP forms at 1.90 and 1.55 A, respectively. The structure was solved by a combination of single-wavelength anomalous dispersion and molecular replacement approaches. The initial search model in the molecular replacement method was built with the result of single-wavelength anomalous dispersion data from the Br-AeKAT crystal in combination with homology modeling. The solved structure shows that the enzyme is a homodimer, and that the two subunits are stabilized by a number of hydrogen bonds, salts bridges, and hydrophobic interactions. Each subunit is divided into an N-terminal arm and small and large domains. Based on its folding, the enzyme belongs to the prototypical fold type, aminotransferase subgroup I. The three-dimensional structure shows a strictly conserved 'PLP-phosphate binding cup' featuring PLP-dependent enzymes. The interaction between Cys284 (A) and Cys284 (B) is unique in AeKAT, which might explain the cysteine effect of AeKAT activity. Further mutation experiments of this residue are needed to eventually understand the mechanism of the enzyme modulation by cysteine.

The role of glutamine transaminase K (GTK) in sulfur and alpha-keto acid metabolism in the brain, and in the possible bioactivation of neurotoxicants

Glutamine transaminase K (GTK), which is a freely reversible glutamine (methionine) aromatic amino acid aminotransferase, is present in most mammalian tissues, including brain. Quantitatively, the most important amine donor in vivo is glutamine. The product of glutamine transamination (i.e., alpha-ketoglutaramate; alphaKGM) is rapidly removed by cyclization and/or conversion to alpha-ketoglutarate. Transamination is therefore "pulled" in the direction of glutamine utilization. Major biological roles of GTK are to maintain low levels of phenylpyruvate and to close the methionine salvage pathway. GTK also catalyzes the transamination of cystathionine, lanthionine, and thialysine to the corresponding alpha-keto acids, which cyclize to ketimines. The cyclic ketimines and several metabolites derived therefrom are found in brain. It is not clear whether these compounds have a biological function or are metabolic dead-ends. However, high-affinity binding of lanthionine ketimine (LK) to brain membranes has been reported. Mammalian tissues possess several enzymes capable of catalyzing transamination of kynurenine in vitro. Two of these kynurenine aminotransferases (KATs), namely KAT I and KAT II, are present in brain and have been extensively studied. KAT I and KAT II are identical to GTK and alpha-aminoadipate aminotransferase, respectively. GTK/KAT I is largely cytosolic in kidney, but mostly mitochondrial in brain. The same gene codes for both forms, but alternative splicing dictates whether a 32-amino acid mitochondrial-targeting sequence is present in the expressed protein. The activity of KAT I is altered by a missense mutation (E61G) in the spontaneously hypertensive rat. The symptoms may be due in part to alteration of kynurenine transamination. However, owing to strong competition from other amino acid substrates, the turnover of kynurenine to kynurenate by GTK/KAT I in nervous tissue must be slow unless kynurenine and GTK are sequestered in a compartment distinct from the major amino acid pools. The possibility is discussed that the spontaneous hypertension in rats carrying the GTK/KAT I mutation may be due in part to disruption of glutamine transamination. GTK is one of several pyridoxal 5'-phosphate (PLP)-containing enzymes that can catalyze non-physiological beta-elimination reactions with cysteine S-conjugates containing a good leaving group attached at the sulfur. These elimination reactions may contribute to the bioactivation of certain electrophiles, resulting in toxicity to kidney, liver, brain, and possibly other organs. On the other hand, the beta-lyase reaction catalyzed by GTK may be useful in the conversion of some cysteine S-conjugate prodrugs to active components in vivo. The roles of GTK in (a) brain nitrogen, sulfur, and aromatic amino acid/kynurenine metabolism, (b) brain alpha-keto acid metabolism, (c) bioactivation of certain electrophiles in brain, (d) prodrug targeting, and (e) maintenance of normal blood pressure deserve further study.

A missense mutation in kynurenine aminotransferase-1 in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) are the most extensively used animal model for genetic hypertension, increased stroke damage, and insulin resistance syndromes; however, the identification of target genes has proved difficult. SHR show elevated sympathetic nerve activity, and stimulation of the central blood pressure control centers with glutamate or nicotine results in exaggerated blood pressure responses, effects that appear to be genetically determined. Kynurenic acid, a competitive glutamate antagonist and a non-competitive nicotinic antagonist, can be synthesized in the brain by the enzyme kynurenine aminotransferase-1 (KAT-1). We have previously shown that KAT-1 activity is significantly reduced in SHR compared with normotensive Wistar Kyoto rats (WKY). Here we show that KAT-1 contains a missense mutation, E61G, in all the strains of SHR examined but not in any of the WKY or outbred strains. Previous studies on F2 rats from a cross of stroke-prone SHR and WKY have shown a suggestive level of linkage between elevated blood pressure and the KAT-1 locus on chromosome 3. In addition, the mutant enzyme expressed in Escherichia coli displays altered kinetics. This mutation may explain the enhanced sensitivity to glutamate and nicotine seen in SHR that may be related to an underlying mechanism of hypertension and increased sensitivity to stroke.

Kynurenines in the CNS: from endogenous obscurity to therapeutic importance

In just under 20 years the kynurenine family of compounds has developed from a group of obscure metabolites of the essential amino acid tryptophan into a source of intensive research, with postulated roles for quinolinic acid in neurodegenerative disorders, most especially the AIDS-dementia complex and Huntington's disease. One of the kynurenines, kynurenic acid, has become a standard tool for use in the identification of glutamate-releasing synapses, and has been used as the parent for several groups of compounds now being developed as drugs for the treatment of epilepsy and stroke. The kynurenines represent a major success in translating a basic discovery into a source of clinical understanding and therapeutic application, with around 3000 papers published on quinolinic acid or kynurenic acid since the discovery of their effects in 1981 and 1982. This review concentrates on some of the recent work most directly relevant to the understanding and applications of kynurenines in medicine.