The kynurenine metabolic pathway in the eye: studies on 3-hydroxykynurenine, a putative cataractogenic compound

Chemical Properties Determine Solubility and Stability in βγ-Crystallins of the Eye Lens

βγ-Crystallins are the primary structural and refractive proteins found in the vertebrate eye lens. Because crystallins are not replaced after early eye development, their solubility and stability must be maintained for a lifetime, which is even more remarkable given the high protein concentration in the lens. Aggregation of crystallins caused by mutations or post-translational modifications can reduce crystallin protein stability and alter intermolecular interactions. Common post-translational modifications that can cause age-related cataracts include deamidation, oxidation, and tryptophan derivatization. Metal ion binding can also trigger reduced crystallin solubility through a variety of mechanisms. Interprotein interactions are critical to maintaining lens transparency: crystallins can undergo domain swapping, disulfide bonding, and liquid-liquid phase separation, all of which can cause opacity depending on the context. Important experimental techniques for assessing crystallin conformation in the absence of a high-resolution structure include dye-binding assays, circular dichroism, fluorescence, light scattering, and transition metal FRET.

Evaluation of the andrographolides role and its indoleamine 2,3-dioxygenase inhibitory potential and attendant molecular mechanism against STZ-induced diabetic rats

The study is to scrutinize andrographolides with Indoleamine 2,3-dioxygenase (IDO) inhibitory potential, its molecular mechanism against streptozotocin (STZ) diabetic retinopathy (DR) in Wistar rats. Oxidative stress markers such as Kynurenine metabolites, retinal histopathological changes have been studied. Further, IDO gene expression and docking studies have been performed. Andrographolide treated rats have been reducing the level of thiobarbituric acid reactive substances and protein carbonyls Kynurenine metabolites with an improvement in the level of GSH and expression of IDO as revealed by morphological changes in inner and outer nuclear layer of the retina. The current results of this study have been generated information about an activity of the andrographolide in the essential pocket of IDO. Our results explain, involving IDO and andrographolide would constitute an attempt to identify natural products with therapeutic value and further studies in this direction would be of immense significance in the administration of diabetes and its related problems.

Discovery and characterization of natural products that act as pheromones in fish

Covering: up to 2018 Fish use a diverse collection of molecules to communicate with conspecifics. Since Karlson and Lüscher termed these molecules 'pheromones', chemists and biologists have joined efforts to characterize their structures and functions. In particular, the understanding of insect pheromones developed at a rapid pace, set, in part, by the use of bioassay-guided fractionation and natural product chemistry. Research on vertebrate pheromones, however, has progressed more slowly. Initially, biologists characterized fish pheromones by screening commercially available compounds suspected to act as pheromones based upon their physiological function. Such biology-driven screening has proven a productive approach to studying pheromones in fish. However, the many functions of fish pheromones and diverse metabolites that fish release make predicting pheromone identity difficult and necessitate approaches led by chemistry. Indeed, the few cases in which pheromone identification was led by natural product chemistry indicated novel or otherwise unpredicted compounds act as pheromones. Here, we provide a brief review of the approaches to identifying pheromones, placing particular emphasis on the promise of using natural product chemistry together with assays of biological activity. Several case studies illustrate bioassay-guided fractionation as an approach to pheromone identification in fish and the unexpected diversity of pheromone structures discovered by natural product chemistry. With recent advances in natural product chemistry, bioassay-guided fractionation is likely to unveil an even broader collection of pheromone structures and enable research that spans across disciplines.

Presence and distribution of L-kynurenine aminotransferases immunoreactivity in human cataractous lenses

PURPOSE

To investigate the presence and distribution of l-kynurenine aminotransferases immunoreactivity in human and animal lenses during cataract formation.

METHODS

Immunohistochemistry was conducted using polyclonal antibodies against KAT I, KAT II and KAT III on sections of 26 anterior capsules from patients undergoing surgical treatment of anterior subcapsular cataract (ASC) and 22 cataractous lenses from human eyes enucleated because of choroidal malignant melanoma. Additionally, the eyes of 11-month-old DBA/2J mice (6 eyes) were investigated (with KAT I and II). Ten clear human lenses and four BL6 mice lenses were used as controls. Spatial immunoreactivity patterns of enzymes were compared with Periodic Acid - Schiff (PAS)-stained sections.

RESULTS

Immunohistochemical analysis revealed presence of KAT I, KAT II and KAT III in extracellular structures of all studied types of cataract in human eyes showing specific pattern of the stain. In cortical cataract, immunoreactivity was observed on cortical lens fibres. In nuclear cataract, KAT II revealed stronger and diffused staining than KAT I. Additionally, both KAT showed more pronounced staining at the edge of small clefts. In normal human lenses, KAT I, II and III, immunoreactivity was not observed. Presence of KAT I and KAT II in the intercellular substance of DBA/2J mice cataract was observed. In BL6 mice lenses without cataract, only weak KAT I and KAT II staining was observed.

CONCLUSIONS

Presence of l-kynurenine aminotransferases in extracellular matrix (ECM) during human cataract formation suggests that products of l-kynurenine pathway might be involved in mechanisms of cataractogenesis.

Kynurenine Pathway in Skin Cells: Implications for UV-Induced Skin Damage

The kynurenine pathway (KP) is the principle route of catabolism of the essential amino acid tryptophan, leading to the production of several neuroactive and immunoregulatory metabolites. Alterations in the KP have been implicated in various neuropsychiatric and neurodegenerative diseases, immunological disorders, and many other diseased states. Although the role of the KP in the skin has been evaluated in small niche fields, limited studies are available regarding the effect of acute ultra violet exposure and the induction of the KP in human skin-derived fibroblasts and keratinocytes. Since UV exposure can illicit an inflammatory component in skin cells, it is highly likely that the KP may be induced in these cells in response to UV exposure. It is also possible that some KP metabolites may act as pro-inflammatory and anti-inflammatory mediators, since the KP is important in immunomodulation.

A Class I UV-blocking (senofilcon A) soft contact lens prevents UVA-induced yellow fluorescence and NADH loss in the rabbit lens nucleus in vivo

It is known that fluorescence, much of it caused by UVA light excitation, increases in the aging human lens, resulting in loss of sharp vision. This study used an in vivo animal model to investigate UVA-excited fluorescence in the rabbit lens, which contains a high level of the UVA chromophore NADH, existing both free and bound to λ-crystallin. Also, the ability of a Class I (senofilcon A) soft contact lens to protect against UVA-induced effects on the rabbit lens was tested. Rabbit eyes were irradiated with UVA light in vivo (100 mW/cm(2) on the cornea) for 1 h using monochromatic 365 nm light. Irradiation was conducted in the presence of either a senofilcon A contact lens, a minimally UV-absorbing lotrafilcon A contact lens, or no contact lens at all. Eyes irradiated without a contact lens showed blue 365 nm-excited fluorescence initially, but this changed to intense yellow fluorescence after 1 h. Isolated, previously irradiated lenses exhibited yellow fluorescence originating from the lens nucleus when viewed under 365 nm light, but showed normal blue fluorescence arising from the cortex. Previously irradiated lenses also exhibited a faint yellow color when observed under visible light. The senofilcon A contact lens protected completely against the UVA-induced effects on fluorescence and lens yellowing, whereas the lotrafilcon A lens showed no protection. The UVA-exposure also produced a 53% loss of total NADH (free plus bound) in the lens nucleus, with only a 13% drop in the anterior cortex. NADH loss in the nucleus was completely prevented with use of a senofilcon A contact lens, but no significant protection was observed with a lotrafilcon A lens. Overall, the senofilcon A lens provided an average of 67% protection against UVA-induced loss of four pyridine nucleotides in four different regions of the lens. HPLC analysis with fluorescence detection indicated a nearly six-fold increase in 365 nm-excited yellow fluorescence arising from lens nuclear λ-crystallin after the in vivo UVA exposure. It is concluded that UVA-induced loss of free NADH (which fluoresces blue) may have allowed the natural yellow fluorescence of λ-crystallin and other proteins in the lens nucleus to become visible. Increased fluorescence exhibited by UVA-exposed λ-crystallin may have been the result of a UVA-induced change in the conformation of the protein occurring during the initial UVA-exposure in vivo. The results demonstrate the greater susceptibility of the lens nucleus to UVA-induced stress, and may relate to the formation of human nuclear cataract. The senofilcon A contact lens was shown to be beneficial in protecting the rabbit lens against effects of UVA light, including changes in fluorescence, increased yellowing and loss of pyridine nucleotides.

A class I (Senofilcon A) soft contact lens prevents UVB-induced ocular effects, including cataract, in the rabbit in vivo

PURPOSE

UVB radiation from sunlight is known to be a risk factor for human cataract. The purpose in this study was to investigate the ability of a class I UV-blocking soft contact lens to protect against UVB-induced effects on the ocular tissues of the rabbit in vivo.

METHODS

Eyes of rabbits were exposed to UVB light for 30 minutes (270-360 nm, peak at 310 nm, 1.7 mW/cm(2) on the cornea). Eyes were irradiated in the presence of either a UV-blocking senofilcon A contact lens, a minimally UV-blocking lotrafilcon A contact lens, or no contact lens at all. Effects on the cornea and lens were evaluated at various times after exposure.

RESULTS

Eyes irradiated with no contact lens protection showed corneal epithelial cell loss plus lens epithelial cell swelling, vacuole formation, and DNA single-strand breaks, as well as lens anterior subcapsular opacification. The senofilcon A lens protected nearly completely against the UVB-induced effects, whereas the lotrafilcon A lens showed no protection.

CONCLUSIONS

The results indicate that use of a senofilcon A contact lens is beneficial in protecting ocular tissues of the rabbit against the harmful effects of UVB light, including photokeratitis and cataract.

Kynurenine-induced photo oxidative damage to lens in vitro: protective effect of caffeine

Photochemical generation of reactive species of oxygen in the lens and aqueous and consequent physiological damage to the tissue has been implicated in the genesis of human cataracts. The present studies were undertaken to examine the feasibility of possible prevention of such damage to the lens initiated by UV activation of kynurenine, a well-known photosensitizer in the human lens. The studies were done by organ culturing intact mouse lenses in medium containing kynurenine and exposed to UVA. Tissue damage was assessed by the inhibition of its ability to carry active transport of rubidium ions and the associated decrements in the levels of GSH and ATP. These deleterious effects were significantly prevented by caffeine, an alkaloid present in many common beverages and known to chemically deactivate the said oxygen derivatives. Further studies on the pharmacological significance of the findings are hence in progress.

Indoleamine 2,3-dioxygenase tissue distribution and cellular localization in mice: implications for its biological functions

Earlier studies have suggested that indoleamine 2,3-dioxygenase (IDO) has a wide tissue distribution in mammals. However, detailed information on its cellular localization and also the levels of expression in various tissues is still scarce. In the present study, we sought to determine the cellular localization of IDO and also to quantify the level of its expression in various mouse tissues by using the branched DNA signal amplification assay, Western blotting, and immunohistochemical staining. The highest levels of constitutive IDO expression were found to be selectively present in the caput of epididymis, except for its initial segment. IDO expression was also detected inside the luminal compartment and even in the stereocilia within this region. In the prostate, high levels of IDO were selectively expressed in the capsular cells. In addition, high levels of IDO expression were also selectively detected in certain types of cells in the placenta, spleen, thymus, lung, and digestive tract. Notably, the morphological features of most of the positively stained cells in these organs closely resembled those of antigen-presenting cells. Based on the tissue distribution and cellular localization characteristics of IDO, it is hypothesized that its expression may serve two main functions: one is to deplete tryptophan in an enclosed microenvironment (such as in the epididymal duct lumen) to prevent bacterial or viral infection, and the other is to produce bioactive tryptophan catabolites that would serve to suppress T-cell-mediated immune responses against self-antigens, fetal antigens, or allogeneic antigens, in different situations.

Elevated concentrations of kynurenic acid, a tryptophan derivative, in dense nuclear cataracts

PURPOSE

Kynurenines and their glycoside derivatives in the ocular lens absorb ultraviolet radiation and thus possibly help protect the retina from ultraviolet light. The current study analysed kynurenine aminotransferase I (KAT I) activity and kynurenic acid (KYNA) concentrations in human senile cataractous lenses and in experimentally induced cataracts in diabetic rats treated with streptozotocin (STZ).

METHODS

KYNA levels and KAT I activity were investigated with HPLC and detected fluorimetrically in the nuclei of 91 human cataractous lenses collected during planned extracapsular extraction. The lenses were classified on the Lens Opacity Classification System III scale and compared with clear lenses regarding KYNA concentrations. Cataractous lenses from STZ-treated rats were compared with control lenses.

RESULTS

KYNA concentration was 0.95 +/- 0.22 in human NC0 (nuclear color) control lenses, 0.8 +/- 0.72 in NC1, 1.18 +/- 0.88 in NC2, 1.31 +/- 0.70 in NC3, 1.78 +/- 0.92 in NC4, 8.80 +/- 8.28 (p < 0.05 vs. NC0) in NC5, and 14.0 +/- 11.1 (p < 0.05 vs. NC0) in NC6. A correlation was found between KYNA concentrations and the grade of cataract (r = 0.047, p < 0.001). KAT I activity in human cataracts was 0.44 +/- 0.16 pmol/mg protein- 1 hr- 1. Elevated KYNA concentrations in rat cataractous lenses were also observed (p < 0.05).

CONCLUSIONS

KYNA levels are elevated in senile nuclear human cataracts and in cataractous lenses of rats with experimentally induced diabetes.

L-Kynurenine, an amino acid identified as a sex pheromone in the urine of ovulated female masu salmon

Many animals employ sex pheromones to find mating partners during their reproductive seasons. However, most sex pheromones of vertebrates remain to be identified. Over the past 20 years, steroids and prostaglandins have been identified as sex pheromones in several fishes. These pheromones are broadly termed "hormonal pheromones" because they or their precursors act as hormones in these fishes. Hitherto, no other type of sex pheromone has been unambiguously identified in teleost fish. Here we report the identification of a "nonhormonal pheromone" in teleost fish. The urine of the reproductively mature female masu salmon (Oncorhynchus masou) contains a male-attracting pheromone. Bioassay-guided fractionation yielded an active compound that was identical to L-kynurenine in spectral and chromatographic properties. L-Kynurenine is a major metabolite of L-tryptophan in vertebrates. This pheromone elicits a male-specific behavior at even picomolar concentrations; its electrophysiological threshold is 10(-14) M. L-Kynurenine is a reasonable substance for female masu salmon to advertise their readiness for mating.

Photochemistry of Kynurenine, a Tryptophan Metabolite: Properties of the Triplet State

Photolysis of aqueous kynurenine (KN) solutions results in the formation of triplet kynurenine TKN. In low pH solutions, triplet formation occurs with almost 100% efficiency, while in neutral solutions the triplet quantum yield is PhiT = 0.018 +/- 0.004. The dissociation constant of TKN, which is attributed to deprotonation of the anilino group, has a pKa value of 4.7. Similar triplet absorption spectra were obtained under direct and acetone-sensitized photolysis. The large difference in quantum yields as a function of pH is attributed to excited-state properties of the first excited singlet state of KN. The rate constant quenching for TKN by oxygen is kq = 2 x 10(9) M(-1) s(-1).

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.

Kynurenine and neopterin in the aqueous humor of the anterior chamber of the eye and in serum of cataract patients

Serum kynurenine, neopterin, selenium concentrations; neopterin concentrations in aqueous humour from anterior chamber of eye and selenium content in lenses were determined in 40 catarat patients with and in 51--without pseudoexfoliation (PES). Significantly increased kynurenine and neopterin levels in serum and neopterin concentration in aqueous humour were observed in mature cataract patients with PES compared to whose without. These patients also presented with the lowest content of selenium in serum and lens, compared with cataract patients without PES. Increased concentrations of neopterin in serum and aqueous humour of anterior chamber of eyes suggest an increased degree of oxidative stress in patients with PES. Thus, the results support the role of oxidative stress in the development of PES in cataract patients. The decreased content of selenium may elicit immune system activation via an increased oxidative stress as it is indicated by the increased formation of kynurenine and neopterin.

Dynamic diversity of the tryptophan pathway in chlamydiae: reductive evolution and a novel operon for tryptophan recapture

BACKGROUND

Complete genomic sequences of closely related organisms, such as the chlamydiae, afford the opportunity to assess significant strain differences against a background of many shared characteristics. The chlamydiae are ubiquitous intracellular parasites that are important pathogens of humans and other organisms. Tryptophan limitation caused by production of interferon-gamma by the host and subsequent induction of indoleamine dioxygenase is a key aspect of the host-parasite interaction. It appears that the chlamydiae have learned to recognize tryptophan depletion as a signal for developmental remodeling. The consequent non-cultivable state of persistence can be increasingly equated to chronic disease conditions.

RESULTS

The genes encoding enzymes of tryptophan biosynthesis were the focal point of this study. Chlamydophila psittaci was found to possess a compact operon containing PRPP synthase, kynureninase, and genes encoding all but the first step of tryptophan biosynthesis. All but one of the genes exhibited translational coupling. Other chlamydiae (Chlamydia trachomatis, C. muridarum and Chlamydophila pneumoniae) lack genes encoding PRPP synthase, kynureninase, and either lack tryptophan-pathway genes altogether or exhibit various stages of reductive loss. The origin of the genes comprising the trp operon does not seem to have been from lateral gene transfer.

CONCLUSIONS

The factors that accommodate the transition of different chlamydial species to the persistent (chronic) state of pathogenesis include marked differences in strategies deployed to obtain tryptophan from host resources. C. psittaci appears to have a novel mechanism for intercepting an early intermediate of tryptophan catabolism and recycling it back to tryptophan. In effect, a host-parasite metabolic mosaic has evolved for tryptophan recycling.

3-Hydroxykynurenine transaminase identity with alanine glyoxylate transaminase. A probable detoxification protein in Aedes aegypti

This study describes the functional characterization of a specific mosquito transaminase responsible for catalyzing the transamination of 3-hydroxykynurenine (3-HK) to xanthurenic acid (XA). The enzyme was purified from Aedes aegypti larvae by ammonium sulfate fractionation, heat treatment, and various chromatographic techniques, plus non-denaturing electrophoresis. The purified transaminase has a relative molecular mass of 42,500 by SDS-PAGE. N-terminal and internal sequencing of the purified protein and its tryptic fragments resolved a partial N-terminal sequence of 19 amino acid residues and 3 partial internal peptide sequences with 7, 10, and 7 amino acid residues. Using degenerate primers based on the partial internal sequences for PCR amplification and cDNA library screening, a full-length cDNA clone with a 1,167-bp open reading frame was isolated. Its deduced amino acid sequence consists of 389 amino acid residues with a predicted molecular mass of 43,239 and shares 45-46% sequence identity with mammalian alanine glyoxylate transaminases. Northern analysis shows the active transcription of the enzyme in larvae and developing eggs. Substrate specificity analysis of this mosquito transaminase demonstrates that the enzyme is active with 3-HK, kynurenine, or alanine substrates. The enzyme has greater affinity and catalytic efficiency for 3-HK than for kynurenine and alanine. The biochemical characteristics of the enzyme in conjunction with the profiles of 3-HK transaminase activity and XA accumulation during mosquito development clearly point out its physiological function in the 3-HK to XA pathway. Our data suggest that the mosquito transaminase was evolved in a manner precisely reflecting the physiological requirement of detoxifying 3-HK produced in the tryptophan oxidation pathway in the mosquito.

Presence of kynurenic acid and kynurenine aminotransferases in the inner retina

Kynurenine aminotransferases (KATs I and II) are pivotal to the synthesis of kynurenic acid (KYNA), the only known endogenous glutamate receptor antagonist and neuroprotectant. This study is the first to identify KYNA in the rat retina and to examine immunohistochemically the distribution of KAT isoforms. As determined by HPLC, KYNA concentration in the retina was 99.9 +/- 24.6 pmol/g wet wt. Immunohisto- chemical experiments showed that both KATs were present in the retina. KAT I was preferentially localised on Müller cell endfeet while KAT II was expressed in cells within the ganglion cell layer. In conclusion, KYNA is present and synthesised in the inner retina. This may suggest a modulatory role in glutamate-mediated retinal neurotransmission.

3-Hydroxykynurenine and 3-hydroxyanthranilic acid generate hydrogen peroxide and promote alpha-crystallin cross-linking by metal ion reduction

The kynurenine pathway catabolite 3-hydroxykynurenine (3HK) and redox-active metals such as copper and iron are implicated in cataractogenesis. Here we investigate the reaction of kynurenine pathway catabolites with copper and iron, as well as interactions with the major lenticular structural proteins, the alpha-crystallins. The o-aminophenol kynurenine catabolites 3HK and 3-hydroxyanthranilic acid (3HAA) reduced Cu(II)>Fe(III) to Cu(I) and Fe(II), respectively, whereas quinolinic acid and the nonphenolic kynurenine catabolites kynurenine and anthranilic acid did not reduce either metal. Both 3HK and 3HAA generated superoxide and hydrogen peroxide in a copper-dependent manner. In addition, 3HK and 3HAA fostered copper-dependent alpha-crystallin cross-linking. 3HK- or 3HAA-modifed alpha-crystallin showed enhanced redox activity in comparison to unmodified alpha-crystallin or ascorbate-modified alpha-crystallin. These data support the possibility that 3HK and 3HAA may be cofactors in the oxidative damage of proteins, such as alpha-crystallin, through interactions with redox-active metals and especially copper. These findings may have relevance for understanding cataractogenesis and other degenerative conditions in which the kynurenine pathway is activated.