Catalytic mechanism and substrate specificity of the beta-subunit of the voltage-gated potassium channel

Insights into Leishmania donovani potassium channel family and their biological functions

Leishmania donovani is the causative organism for visceral leishmaniasis. Although this parasite was discovered over a century ago, nothing is known about role of potassium channels in L. donovani. Potassium channels are known for their crucial roles in cellular functions in other organisms. Recently the presence of a calcium-activated potassium channel in L. donovani was reported which prompted us to look for other proteins which could be potassium channels and to investigate their possible physiological roles. Twenty sequences were identified in L. donovani genome and subjected to estimation of physio-chemical properties, motif analysis, localization prediction and transmembrane domain analysis. Structural predictions were also done. The channels were majorly α-helical and predominantly localized in cell membrane and lysosomes. The signature selectivity filter of potassium channel was present in all the sequences. In addition to the conventional potassium channel activity, they were associated with gene ontology terms for mitotic cell cycle, cell death, modulation by virus of host process, cell motility etc. The entire study indicates the presence of potassium channel families in L. donovani which may have involvement in several cellular pathways. Further investigations on these putative potassium channels are needed to elucidate their roles in Leishmania.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03692-y.

Basal NAD(H) redox state permits hydrogen peroxide-induced mesenteric artery dilatation

Smooth muscle voltage-gated K+ (Kv) channels in resistance arteries control vascular tone and contribute to the coupling of blood flow with local metabolic activity. Members of the Kv1 family are expressed in vascular smooth muscle and are modulated upon physiological elevation of local metabolites, including the glycolytic end-product l-lactate and superoxide-derived hydrogen peroxide (H2 O2 ). Here, we show that l-lactate elicits vasodilatation of small-diameter mesenteric arteries in a mechanism that requires lactate dehydrogenase (LDH). Using the inside-out configuration of the patch clamp technique, we show that increases in NADH that reflect LDH-mediated conversion of l-lactate to pyruvate directly stimulate the activity of single Kv1 channels and significantly enhance the sensitivity of Kv1 activity to H2 O2 . Consistent with these findings, H2 O2 -evoked vasodilatation was significantly greater in the presence of 10 mM l-lactate relative to lactate-free conditions, yet was abolished in the presence of 10 mM pyruvate, which shifts the LDH reaction towards the generation of NAD+ . Moreover, the enhancement of H2 O2 -induced vasodilatation was abolished in arteries from double transgenic mice with selective overexpression of the intracellular Kvβ1.1 subunit in smooth muscle cells. Together, our results indicate that the Kvβ complex of native vascular Kv1 channels serves as a nodal effector for multiple redox signals to precisely control channel activity and vascular tone in the face of dynamic tissue-derived metabolic cues. KEY POINTS: Vasodilatation of mesenteric arteries by elevated external l-lactate requires its conversion by lactate dehydrogenase. Application of either NADH or H2 O2 potentiates single Kv channel currents in excised membrane patches from mesenteric artery smooth muscle cells. The binding of NADH enhances the stimulatory effects of H2 O2 on single Kv channel activity. The vasodilatory response to H2 O2 is differentially modified upon elevation of external l-lactate or pyruvate. The presence of l-lactate enhances the vasodilatory response to H2 O2 via the Kvβ subunit complex in smooth muscle.

Genetic deletion of Kvβ2 (AKR6) causes loss of muscle function and increased inflammation in mice

The voltage-gated potassium channels (Kv) are complex ion channels with distinct roles in neurotransmission, electrical conductivity of the heart, and smooth and striated muscle functions. Previously, we demonstrated that deletion of Kvβ2 in mice results in decreased Pax7 protein levels, hindlimb muscles and body weights, and fiber type switching. In the present study, we tested the hypothesis that Kvβ2 regulates skeletal muscle function in mice. The young and old Kvβ2 knockout (KO) and wildtype (WT) mice were utilized to test the aging phenotype and skeletal muscle function. Consistent with our previous finding, we found a significant reduction in hindlimb skeletal muscles mass and body weight in young Kvβ2 KO mice, which was also significantly reduced in old Kvβ2 KO mice compared with age-matched WT mice. Forelimb grip strength, and the hindleg extensor digitorum longus (EDL) muscles force-frequency relations were significantly decreased in young and old Kvβ2 KO mice compared to age-matched WT mice. Analysis of transmission electron microscopy images of EDL muscles in young mice revealed a significant reduction in the sarcomere length for Kvβ2 KO vs. WT. Hematoxylin and eosin-stained tibialis anterior muscles cryosections displayed a significant decrease in the number of medium (2,000-4,000 µm2) and largest (>4,000 µm2) myofibers area in young Kvβ2 KO vs. WT mice. We also found a significant increase in fibrotic tissue area in young Kvβ2 KO mice compared with age-matched WT mice. Analysis of RNA Seq data of the gastrocnemius muscles (GAS) identified significant increase in genes involved in skeletal muscle development, proliferation and cell fate determination, atrophy, energy metabolism, muscle plasticity, inflammation, and a decrease in circadian core clock genes in young Kvβ2 KO vs. WT mice. Several genes were significantly upregulated (384 genes) and downregulated (40 genes) in young Kvβ2 KO mice compared to age-matched WT mice. Further, RT-qPCR analysis of the GAS muscles displayed a significant increase in pro-inflammatory marker Il6 expression in young Kvβ2 KO mice compared to age-matched WT mice. Overall, the present study shows that deletion of Kvβ2 leads to decreased muscles strength and increased inflammation.

Structure-function characterization of an aldo-keto reductase involved in detoxification of the mycotoxin, deoxynivalenol

Deoxynivalenol (DON) is a mycotoxin, produced by filamentous fungi such as Fusarium graminearum, that causes significant yield losses of cereal grain crops worldwide. One of the most promising methods to detoxify this mycotoxin involves its enzymatic epimerization to 3-epi-DON. DepB plays a critical role in this process by reducing 3-keto-DON, an intermediate in the epimerization process, to 3-epi-DON. DepB_Rleg from Rhizobium leguminosarum is a member of the new aldo-keto reductase family, AKR18, and it has the unusual ability to utilize both NADH and NADPH as coenzymes, albeit with a 40-fold higher catalytic efficiency with NADPH compared to NADH. Structural analysis of DepB_Rleg revealed the putative roles of Lys-217, Arg-290, and Gln-294 in NADPH specificity. Replacement of these residues by site-specific mutagenesis to negatively charged amino acids compromised NADPH binding with minimal effects on NADH binding. The substrate-binding site of DepB_Rleg is larger than its closest structural homolog, AKR6A2, likely contributing to its ability to utilize a wide range of aldehydes and ketones, including the mycotoxin, patulin, as substrates. The structure of DepB_Rleg also suggests that 3-keto-DON can adopt two binding modes to facilitate 4-pro-R hydride transfer to either the re- or si-face of the C3 ketone providing a possible explanation for the enzyme's ability to convert 3-keto-DON to 3-epi-DON and DON in diastereomeric ratios of 67.2% and 32.8% respectively.

Diversification of Potassium Currents in Excitable Cells via Kvβ Proteins

Excitable cells of the nervous and cardiovascular systems depend on an assortment of plasmalemmal potassium channels to control diverse cellular functions. Voltage-gated potassium (Kv) channels are central to the feedback control of membrane excitability in these processes due to their activation by depolarized membrane potentials permitting K⁺ efflux. Accordingly, Kv currents are differentially controlled not only by numerous cellular signaling paradigms that influence channel abundance and shape voltage sensitivity, but also by heteromeric configurations of channel complexes. In this context, we discuss the current knowledge related to how intracellular Kvβ proteins interacting with pore complexes of Shaker-related Kv1 channels may establish a modifiable link between excitability and metabolic state. Past studies in heterologous systems have indicated roles for Kvβ proteins in regulating channel stability, trafficking, subcellular targeting, and gating. More recent works identifying potential in vivo physiologic roles are considered in light of these earlier studies and key gaps in knowledge to be addressed by future research are described.

Pyridine nucleotide redox potential in coronary smooth muscle couples myocardial blood flow to cardiac metabolism

Adequate oxygen delivery to the heart during stress is essential for sustaining cardiac function. Acute increases in myocardial oxygen demand evoke coronary vasodilation and enhance perfusion via functional upregulation of smooth muscle voltage-gated K⁺ (Kv) channels. Because this response is controlled by Kv1 accessory subunits (i.e., Kvβ), which are NAD(P)(H)-dependent aldo-keto reductases, we tested the hypothesis that oxygen demand modifies arterial [NAD(H)]_i, and that resultant cytosolic pyridine nucleotide redox state influences Kv1 activity. High-resolution imaging mass spectrometry and live-cell imaging reveal cardiac workload-dependent increases in NADH:NAD⁺ in intramyocardial arterial myocytes. Intracellular NAD(P)(H) redox ratios reflecting elevated oxygen demand potentiate native coronary Kv1 activity in a Kvβ2-dependent manner. Ablation of Kvβ2 catalysis suppresses redox-dependent increases in Kv1 activity, vasodilation, and the relationship between cardiac workload and myocardial blood flow. Collectively, this work suggests that the pyridine nucleotide sensitivity and enzymatic activity of Kvβ2 controls coronary vasoreactivity and myocardial blood flow during metabolic stress.

Physiological matching of blood flow to the demand for oxygen by the heart is required for sustained cardiac health, yet the underlying mechanisms are obscure. Here, the authors report a key role for acute modifications to the redox state of intracellular pyridine nucleotides in coronary smooth muscle and their impact on voltage-gated K + channels in metabolic vasodilation

S-acylation-dependent membrane microdomain localization of the regulatory Kvβ2.1 subunit

The voltage-dependent potassium (Kv) channel Kvβ family was the first identified group of modulators of Kv channels. Kvβ regulation of the α-subunits, in addition to their aldoketoreductase activity, has been under extensive study. However, scarce information about their specific α-subunit-independent biology is available. The expression of Kvβs is ubiquitous and, similar to Kv channels, is tightly regulated in leukocytes. Although Kvβ subunits exhibit cytosolic distribution, spatial localization, in close contact with plasma membrane Kv channels, is crucial for a proper immune response. Therefore, Kvβ2.1 is located near cell surface Kv1.3 channels within the immunological synapse during lymphocyte activation. The objective of this study was to analyze the structural elements that participate in the cellular distribution of Kvβs. It was demonstrated that Kvβ peptides, in addition to the cytoplasmic pattern, targeted the cell surface in the absence of Kv channels. Furthermore, Kvβ2.1, but not Kvβ1.1, targeted lipid raft microdomains in an S-acylation-dependent manner, which was concomitant with peptide localization within the immunological synapse. A pair of C-terminal cysteines (C301/C311) was mostly responsible for the specific palmitoylation of Kvβ2.1. Several insults altered Kvβ2.1 membrane localization. Therefore, growth factor-dependent proliferation enhanced surface targeting, whereas PKC activation impaired lipid raft expression. However, PSD95 stabilized Kvβ2.1 in these domains. This data shed light on the molecular mechanism by which Kvβ2.1 clusters into immunological synapses during leukocyte activation.

Non-P450 Drug-Metabolizing Enzymes: Contribution to Drug Disposition, Toxicity, and Development

Most clinically used drugs are metabolized in the body via oxidation, reduction, or hydrolysis reactions, which are considered phase I reactions. Cytochrome P450 (P450) enzymes, which primarily catalyze oxidation reactions, contribute to the metabolism of over 50% of clinically used drugs. In the last few decades, the function and regulation of P450s have been extensively studied, whereas the characterization of non-P450 phase I enzymes is still incomplete. Recent studies suggest that approximately 30% of drug metabolism is carried out by non-P450 enzymes. This review summarizes current knowledge of non-P450 phase I enzymes, focusing on their roles in controlling drug efficacy and adverse reactions as an important aspect of drug development. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Protective Effect of Aldo-keto Reductase 1B1 Against Neuronal Cell Damage Elicited by 4'-Fluoro-α-pyrrolidinononanophenone

Chronic exposure to cathinone derivatives increases the risk of severe health hazards, whereas little is known about the detailed pathogenic mechanisms triggered by the derivatives. We have recently shown that treatment with α-pyrrolidinononanophenone (α-PNP, a highly lipophilic cathinone derivative possessing a long hydrocarbon main chain) provokes neuronal cell apoptosis and its 4'-fluorinated analog (F-α-PNP) potently augments the apoptotic effect. In this study, we found that neuronal SK-N-SH cell damage elicited by F-α-PNP treatment is augmented most potently by pre-incubation with an AKR1B1 inhibitor tolrestat, among specific inhibitors of four aldo-keto reductase (AKR) family members (1B1, 1C1, 1C2, and 1C3) expressed in the neuronal cells. In addition, forced overexpression of AKR1B1 remarkably lowered the cell sensitivity to F-α-PNP toxicity, clearly indicating that AKR1B1 protects from neurotoxicity of the derivative. Treatment of SK-N-SH cells with F-α-PNP resulted in a dose-dependent up-regulation of AKR1B1 expression and activation of its transcription factor NF-E2-related factor 2. Metabolic analyses using liquid chromatography/mass spectrometry/mass spectrometry revealed that AKR1B1 is hardly involved in the F-α-PNP metabolism. The F-α-PNP treatment resulted in production of reactive oxygen species and lipid peroxidation byproduct 4-hydroxy-2-nonenal (HNE) in the cells. The enhanced HNE level was reduced by overexpression of AKR1B1, which also lessened the cell damage elicited by HNE. These results suggest that the AKR1B1-mediated neuronal cell protection is due to detoxification of HNE formed by F-α-PNP treatment, but not to metabolism of the derivative.

Deletion of Kvβ2 (AKR6) Attenuates Isoproterenol Induced Cardiac Injury with Links to Solute Carrier Transporter SLC41a3 and Circadian Clock Genes

Kvβ subunits belong to the aldo-keto reductase superfamily, which plays a significant role in ion channel regulation and modulates the physiological responses. However, the role of Kvβ2 in cardiac pathophysiology was not studied, and therefore, in the present study, we hypothesized that Kvβ2 plays a significant role in cardiovascular pathophysiology by modulating the cardiac excitability and gene responses. We utilized an isoproterenol-infused mouse model to investigate the role of Kvβ2 and the cardiac function, biochemical changes, and molecular responses. The deletion of Kvβ2 attenuated the QTc (corrected QT interval) prolongation at the electrocardiographic (ECG) level after a 14-day isoproterenol infusion, whereas the QTc was significantly prolonged in the littermate wildtype group. Monophasic action potentials verified the ECG changes, suggesting that cardiac changes and responses due to isoproterenol infusion are mediated similarly at both the in vivo and ex vivo levels. Moreover, the echocardiographic function showed no further decrease in the ejection fraction in the isoproterenol-stimulated Kvβ2 knockout (KO) group, whereas the wildtype mice showed significantly decreased function. These experiments revealed that Kvβ2 plays a significant role in cardiovascular pathophysiology. Furthermore, the present study revealed SLC41a3, a major solute carrier transporter affected with a significantly decreased expression in KO vs. wildtype hearts. The electrical function showed that the decreased expression of SLC41a3 in Kvβ2 KO hearts led to decreased Mg²⁺ responses, whereas, in the wildtype hearts, Mg²⁺ caused action potential duration (APD) shortening. Based on the in vivo, ex vivo, and molecular evaluations, we identified that the deletion of Kvβ2 altered the cardiac pathophysiology mediated by SLC41a3 and altered the NAD (nicotinamide adenine dinucleotide)-dependent gene responses.

Myocardial Blood Flow Control by Oxygen Sensing Vascular Kvβ Proteins

[Figure: see text].

Quantitative analysis of mRNA expression levels of aldo-keto reductase and short-chain dehydrogenase/reductase isoforms in human livers

The aldo-keto reductase (AKR) and short-chain dehydrogenase/reductase (SDR) superfamilies are responsible for the reduction in compounds containing the aldehyde, ketone, and quinone groups. In humans, 12 AKR isoforms (AKR1A1, AKR1B1, AKR1B10, AKR1B15, AKR1C1, AKR1C2, AKR1C3, AKR1C4, AKR1D1, AKR1E2, AKR7A2, and AKR7A3) and 6 SDR isoforms (CBR1, CBR3, CBR4, HSD11B1, DHRS4, and DCXR) have been found to catalyze the reduction in xenobiotics, but their hepatic expression levels are unclear. The purpose of this study is to determine the absolute mRNA expression levels of these 18 isoforms in the human liver. In 22 human livers, all isoforms, except for AKR1B15, are expressed, and AKR1C2 (on average 1.6 × 10⁶ copy/μg total RNA), AKR1C3 (1.3 × 10⁶), AKR1C1 (1.3 × 10⁶), CBR1 (9.7 × 10⁵), and HSD11B1 (1.1 × 10⁶) are abundant, representing 67% of the total expression of reductases in the liver. The expression levels of AKR1C2, AKR1C3, AKR1C1, CBR1, and HSD11B1 are significantly correlated with each other, except between AKR1C2 and CBR1, suggesting that they might be regulated by common factor(s). In conclusion, this study comprehensively determined the absolute expression of mRNA expression of each AKR and SDR isoform in the human liver.

The Biosynthesis of Enzymatically Oxidized Lipids

Enzymatically oxidized lipids are a specific group of biomolecules that function as key signaling mediators and hormones, regulating various cellular and physiological processes from metabolism and cell death to inflammation and the immune response. They are broadly categorized as either polyunsaturated fatty acid (PUFA) containing (free acid oxygenated PUFA "oxylipins", endocannabinoids, oxidized phospholipids) or cholesterol derivatives (oxysterols, steroid hormones, and bile acids). Their biosynthesis is accomplished by families of enzymes that include lipoxygenases (LOX), cyclooxygenases (COX), cytochrome P450s (CYP), and aldo-keto reductases (AKR). In contrast, non-enzymatically oxidized lipids are produced by uncontrolled oxidation and are broadly considered to be harmful. Here, we provide an overview of the biochemistry and enzymology of LOXs, COXs, CYPs, and AKRs in humans. Next, we present biosynthetic pathways for oxylipins, oxidized phospholipids, oxysterols, bile acids and steroid hormones. Last, we address gaps in knowledge and suggest directions for future work.

Metabolic regulation of Kv channels and cardiac repolarization by Kvβ2 subunits

Voltage-gated potassium (Kv) channels control myocardial repolarization. Pore-forming Kvα proteins associate with intracellular Kvβ subunits, which bind pyridine nucleotides with high affinity and differentially regulate channel trafficking, plasmalemmal localization and gating properties. Nevertheless, it is unclear how Kvβ subunits regulate myocardial K⁺ currents and repolarization. Here, we tested the hypothesis that Kvβ2 subunits regulate the expression of myocardial Kv channels and confer redox sensitivity to Kv current and cardiac repolarization. Co-immunoprecipitation and in situ proximity ligation showed that in cardiac myocytes, Kvβ2 interacts with Kv1.4, Kv1.5, Kv4.2, and Kv4.3. Cardiac myocytes from mice lacking Kcnab2 (Kvβ2^-/-) had smaller cross sectional areas, reduced sarcolemmal abundance of Kvα binding partners, reduced I_to, I_K,slow1, and I_K,slow2 densities, and prolonged action potential duration compared with myocytes from wild type mice. These differences in Kvβ2^-/- mice were associated with greater P wave duration and QT interval in electrocardiograms, and lower ejection fraction, fractional shortening, and left ventricular mass in echocardiographic and morphological assessments. Direct intracellular dialysis with a high NAD(P)H:NAD(P)⁺ accelerated Kv inactivation in wild type, but not Kvβ2^-/- myocytes. Furthermore, elevated extracellular levels of lactate increased [NADH]_i and prolonged action potential duration in wild type cardiac myocytes and perfused wild type, but not Kvβ2^-/-, hearts. Taken together, these results suggest that Kvβ2 regulates myocardial electrical activity by supporting the functional expression of proteins that generate I_to and I_K,slow, and imparting redox and metabolic sensitivity to Kv channels, thereby coupling cardiac repolarization to myocyte metabolism.

Discovery of pyridoxal reductase activity as part of human vitamin B6 metabolism

BACKGROUND

Pyridoxal 5'-phosphate (PLP) is the active form of vitamin B6. Mammals cannot synthesize vitamin B6, so they rely on dietary uptake of the different B6 forms, and via the B6 salvage pathway they interconvert them into PLP. Humans possess three enzymes in this pathway: pyridoxal kinase, pyridox(am)ine phosphate oxidase and pyridoxal phosphatase. Besides these, a fourth enzyme has been described in plants and yeast but not in humans: pyridoxal reductase.

METHODS

We analysed B6 vitamers in remnant CSF samples of PLP-treated patients and four mammalian cell lines (HepG2, Caco2, HEK293 and Neuro-2a) supplemented with PL as the sole source of vitamin B6.

RESULTS

Strong accumulation of pyridoxine (PN) in CSF of PLP-treated patients was observed, suggesting the existence of a PN-forming enzyme. Our in vitro studies show that all cell lines reduce PL to PN in a time- and dose-dependent manner. We compared the amino acid sequences of known PL reductases to human sequences and found high homology for members of the voltage-gated potassium channel beta subunits and the human aldose reductases. Pharmacological inhibition and knockout of these proteins show that none of the candidates is solely responsible for PL reduction to PN.

CONCLUSIONS

We show evidence for the presence of PL reductase activity in humans. Further studies are needed to identify the responsible protein.

GENERAL SIGNIFICANCE

This study expands the number of enzymes with a role in B6 salvage pathway. We hypothesize a protective role of PL reductase(s) by limiting the intracellular amount of free PL and PLP.

Biochemical and physiological properties of K+ channel-associated AKR6A (Kvβ) proteins

Voltage-gated potassium (Kv) channels play an essential role in the regulation of membrane excitability and thereby control physiological processes such as cardiac excitability, neural communication, muscle contraction, and hormone secretion. Members of the Kv1 and Kv4 families are known to associate with auxiliary intracellular Kvβ subunits, which belong to the aldo-keto reductase superfamily. Electrophysiological studies have shown that these proteins regulate the gating properties of Kv channels. Although the three gene products encoding Kvβ proteins are functional enzymes in that they catalyze the nicotinamide adenine dinucleotide phosphate (NAD[P]H)-dependent reduction of a wide range of aldehyde and ketone substrates, the physiological role for these proteins and how each subtype may perform unique roles in coupling membrane excitability with cellular metabolic processes remains unclear. Here, we discuss current knowledge of the enzymatic properties of Kvβ proteins from biochemical studies with their described and purported physiological and pathophysiological influences.

A potassium channel β-subunit couples mitochondrial electron transport to sleep

The essential but enigmatic functions of sleep^1,2 must be reflected in molecular changes sensed by the brain's sleep-control systems. In the fruitfly Drosophila, about two dozen sleep-inducing neurons³ with projections to the dorsal fan-shaped body (dFB) adjust their electrical output to sleep need⁴, via the antagonistic regulation of two potassium conductances: the leak channel Sandman imposes silence during waking, whereas increased A-type currents through Shaker support tonic firing during sleep⁵. Here we show that oxidative byproducts of mitochondrial electron transport^6,7 regulate the activity of dFB neurons through a nicotinamide adenine dinucleotide phosphate (NADPH) cofactor bound to the oxidoreductase domain^8,9 of Shaker's K_Vβ subunit, Hyperkinetic^10,11. Sleep loss elevates mitochondrial reactive oxygen species in dFB neurons, which register this rise by converting Hyperkinetic to the NADP⁺-bound form. The oxidation of the cofactor slows the inactivation of the A-type current and boosts the frequency of action potentials, thereby promoting sleep. Energy metabolism, oxidative stress, and sleep-three processes implicated independently in lifespan, ageing, and degenerative disease^6,12-14-are thus mechanistically connected. K_Vβ substrates^8,15,16 or inhibitors that alter the ratio of bound NADPH to NADP⁺ (and hence the record of sleep debt or waking time) represent prototypes of potential sleep-regulatory drugs.

Coronary microvascular Kv1 channels as regulatory sensors of intracellular pyridine nucleotide redox potential

Smooth muscle voltage-gated potassium (Kv) channels are important regulators of microvascular tone and tissue perfusion. Recent studies indicate that Kv1 channels represent a key component of the physiological coupling between coronary blood flow and myocardial oxygen demand. While the mechanisms by which metabolic changes in the heart are transduced to alter coronary Kv1 channel gating and promote vasodilation are unclear, a growing body of evidence underscores a pivotal role of Kv1 channels in sensing the cellular redox status. Here, we discuss current knowledge of mechanisms of Kv channel redox regulation with respect to pyridine nucleotide modulation of Kv1 function via ancillary Kvβ proteins as well as direct modulation of channel activity via reactive oxygen and nitrogen species. We identify areas of additional research to address the integration of regulatory processes under altered physiological and pathophysiological conditions that may reveal insights into novel treatment strategies for conditions in which the matching of coronary blood supply and myocardial oxygen demand is compromised.

Corticosteroids and aldose reductase inhibitor Epalrestat modulates cardiac action potential via Kvβ1.1 (AKR6A8) subunit of voltage-gated potassium channel

We previously demonstrated the role of Kvβ1.1 subunit of voltage-activated potassium channel in heart for its sensory roles in detecting changes in NADH/NAD and modulation of ion channel. However, the pharmacological role for the association of Kvβ1 via its binding to ligands such as cortisone and its analogs remains unknown. Therefore, we investigated the significance of Kvβ1.1 binding to cortisone analogs and AR inhibitor epalrestat. In addition, the aldose reductase (AR) inhibitor epalrestat was identified as a pharmacological target and modulator of cardiac activity via binding to the Kvβ1 subunit. Using a combination of ex vivo cardiac electrophysiology and in silico binding, we identified that Kvβ1 subunit binds and interacts with epalrestat. To identify the specificity of the action potential changes, we studied the sensitivity of the action potential prolongation by probing the electrical changes in the presence of 4-aminopyridine and evaluated the specificity of pharmacological effects in the hearts from Kvβ1.1 knock out mouse. Our results show that pharmacological modulation of cardiac electrical activity by cortisone analogs and epalrestat is mediated by Kvβ1.1.

Heteromeric complexes of aldo-keto reductase auxiliary KVβ subunits (AKR6A) regulate sarcolemmal localization of KV1.5 in coronary arterial myocytes

Redox-sensitive potassium channels consisting of the voltage-gated K⁺ (K_V) channel pore subunit K_V1.5 regulate resting membrane potential and thereby contractility of vascular smooth muscle cells. Members of the K_V1 family associate with cytosolic auxiliary β subunits, which are members of the aldo-keto reductase (AKR) superfamily (AKR6A subfamily). The Kvβ subunits have been proposed to regulate Kv1 gating via pyridine nucleotide cofactor binding. However, the molecular identity of K_Vβ subunits that associate with native K_V1.5 channels in the vasculature is unknown. Here, we examined mRNA and protein expression of K_Vβ subunits and tested whether K_Vβ isoforms interact with K_V1.5 channels in murine coronary arteries. We detected K_Vβ1 (AKR6A3), K_Vβ2 (AKR6A5) and K_Vβ3 (AKR6A9) transcripts and K_Vβ1 and K_Vβ2 protein in left anterior descending coronary arteries by real time quantitative PCR and Western blot, respectively. In situ proximity ligation assays indicated abundant protein-protein interactions between K_V1.5/K_Vβ1, K_V1.5/K_Vβ2 and K_Vβ1/β2 in coronary arterial myocytes. Confocal microscopy and membrane fractionation analyses suggest that arterial myocytes from K_Vβ2-null mice have reduced abundance of sarcolemmal K_V1.5. Together, data suggest that in coronary arterial myocytes, K_V1.5 channels predominantly associate with K_Vβ1 and K_Vβ2 proteins and that K_Vβ2 performs a chaperone function for K_V1.5 channels in arterial myocytes, thereby facilitating Kv1α trafficking and membrane localization.

Fly DPP10 acts as a channel ancillary subunit and possesses peptidase activity

Mammalian DPP6 (DPPX) and DPP10 (DPPY) belong to a family of dipeptidyl peptidases, but lack enzyme activity. Instead, these proteins form complexes with voltage-gated K(+) channels in Kv4 family to control their gating and other properties. Here, we find that the fly DPP10 ortholog acts as an ancillary subunit of Kv4 channels and digests peptides. Similarly to mammalian DPP10, the fly ortholog tightly binds to rat Kv4.3 protein. The association causes negative shifts in voltage dependence of channel activation and steady state inactivation. It also results in faster inactivation and recovery from inactivation. In addition to its channel regulatory role, fly DPP10 exhibits significant dipeptidyl peptidase activity with Gly-Pro-MCA (glycyl-L-proline 4-methylcoumaryl-7-amide) as a substrate. Heterologously expressed Flag-tagged fly DPP10 and human DPP4 show similar Km values towards this substrate. However, fly DPP10 exhibits approximately a 6-times-lower relative kcat value normalized with anti-Flag immunoreactivity than human DPP4. These results demonstrate that fly DPP10 is a dual functional protein, controlling Kv4 channel gating and removing bioactive peptides.

Pharmacogenomics of cardiovascular complications in diabetes and obesity

Heart disease is a major cause of death in US and worldwide. The complex interplay of the mechanisms between diabetes, obesity and inflammation raises concerns for therapeutic understanding and developing treatment options for patients. Recent advances utilizing pharmacogenomics has helped researchers to probe in to disease pathophysiology and physicians to detect and, diagnose the disease in patients. The understanding developed in the area primarily addresses the issue focusing on the nature and asks the question 'Why' some individuals respond to the standard medication regimen and others do not. The central idea that genomics play a vital part in how the healthcare providers: physician, pharmacist, and nurse provide treatment utilizing the best practices available for maximum benefits. Pharmacogenomics is the scientific basis which offers the fundamental understanding for diseases, based on which therapeutic approaches can be designed and delivered. The discovery that not all humans respond to the drug in the same way is a 'paradigm shift' in how current therapies are offered. The area of pharmacogenomics at its core is linked to the genetic basis for the disease and the response to treatment. Given that diabetes and obesity are major metabolic ailments globally wherein patients also often suffer from cardiac disorders, a comprehensive genetic and pharmacogenomic understanding of these conditions enable the development of effective therapeutic strategies. In this review, we discuss various pharmacogenomic approaches with special emphasis on heart disease as it relates to diabetes and obesity. Recent information in regard to relevant patents in this topic are also discussed.

Oxidative and reductive metabolism of lipid-peroxidation derived carbonyls

Extensive research has shown that increased production of reactive oxygen species (ROS) results in tissue injury under a variety of pathological conditions and chronic degenerative diseases. While ROS are highly reactive and can incite significant injury, polyunsaturated lipids in membranes and lipoproteins are their main targets. ROS-triggered lipid-peroxidation reactions generate a range of reactive carbonyl species (RCS), and these RCS spread and amplify ROS-related injury. Several RCS generated in oxidizing lipids, such as 4-hydroxy trans-2-nonenal (HNE), 4-oxo-2-(E)-nonenal (ONE), acrolein, malondialdehyde (MDA) and phospholipid aldehydes have been shown to be produced under conditions of oxidative stress and contribute to tissue injury and dysfunction by depleting glutathione and other reductants leading to the modification of proteins, lipids, and DNA. To prevent tissue injury, these RCS are metabolized by several oxidoreductases, including members of the aldo-keto reductase (AKR) superfamily, aldehyde dehydrogenases (ALDHs), and alcohol dehydrogenases (ADHs). Metabolism via these enzymes results in RCS inactivation and detoxification, although under some conditions, it can also lead to the generation of signaling molecules that trigger adaptive responses. Metabolic transformation and detoxification of RCS by oxidoreductases prevent indiscriminate ROS toxicity, while at the same time, preserving ROS signaling. A better understanding of RCS metabolism by oxidoreductases could lead to the development of novel therapeutic interventions to decrease oxidative injury in several disease states and to enhance resistance to ROS-induced toxicity.

New inhibitors of the Kvβ2 subunit from mammalian Kv1 potassium channels

The role of the redox state of Kvβ subunits in the modulation of Kv1 potassium channels has been well documented over the past few years. It has been suggested that a molecule that binds to or inhibits the aldo-keto reductase activity of Kvβ might affect the modulation of channel properties. Previous studies of possible modulators of channel activity have shown that cortisone and some related compounds are able to physically dissociate the channel components by binding to a site at the interface between α and β subunits. Herein, we describe some new inhibitors of rat brain Kvβ2, identified using an assay based on multiple substrate turnover. This approach allows one to focus on molecules that specifically block NADPH oxidation. These studies showed that, at 0.5mM, 3,4-dihydroxphenylacetic acid (DOPAC) was an inhibitor of Kvβ2 turnover yielding a ∼ 40-50% reduction in the aldehyde reductase activity of this subunit. Other significant inhibitors include the bioflavinoid, rutin and the polyphenol resveratrol; some of the known cardioprotective effects of these molecules may be attributable to Kv1 channel modulation. Cortisone or catechol caused moderate inhibition of Kvβ2 turnover, and the aldo-keto reductases inhibitor valproate had an even smaller effect. Despite the importance of the Kv1 channels in a number of disease states, there have been few Kvβ2 inhibitors reported. While the ones identified in this study are only effective at high concentrations, they could serve as tools to decipher the role of Kvβ2 in vivo and, eventually, inform the development of novel therapeutics.

Sequestosome1/p62: a regulator of redox-sensitive voltage-activated potassium channels, arterial remodeling, inflammation, and neurite outgrowth

Sequestosome1/p62 (SQSTM1) is an oxidative stress-inducible protein regulated by the redox-sensitive transcription factor Nrf2. It is not an antioxidant but known as a multifunctional regulator of cell signaling with an ability to modulate targeted or selective degradation of proteins through autophagy. SQSTM1 implements these functions through physical interactions with different types of proteins including atypical PKCs, nonreceptor-type tyrosine kinase p56(Lck) (Lck), polyubiquitin, and autophagosomal factor LC3. One of the notable physiological functions of SQSTM1 is the regulation of redox-sensitive voltage-gated potassium (Kv) channels which are composed of α and β subunits: (Kvα)4 (Kvβ)4. Previous studies have established that SQSTM1 scaffolds PKCζ, enhancing phosphorylation of Kvβ which induces inhibition of pulmonary arterial Kv1.5 channels under acute hypoxia. Recent studies reveal that Lck indirectly interacts with Kv1.3 α subunits and plays a key role in acute hypoxia-induced Kv1.3 channel inhibition in T lymphocytes. Kv1.3 channels provide a signaling platform to modulate the migration and proliferation of arterial smooth muscle cells and activation of T lymphocytes, and hence have been recognized as a therapeutic target for treatment of restenosis and autoimmune diseases. In this review, we focus on the functional interactions of SQSTM1 with Kv channels through two key partners aPKCs and Lck. Furthermore, we provide molecular insights into the functions of SQSTM1 in suppression of proliferation of arterial smooth muscle cells and neointimal hyperplasia following carotid artery ligation, in T lymphocyte differentiation and activation, and in NGF-induced neurite outgrowth in PC12 cells.

Regulation of ion channels by pyridine nucleotides

Recent research suggests that in addition to their role as soluble electron carriers, pyridine nucleotides [NAD(P)(H)] also regulate ion transport mechanisms. This mode of regulation seems to have been conserved through evolution. Several bacterial ion-transporting proteins or their auxiliary subunits possess nucleotide-binding domains. In eukaryotes, the Kv1 and Kv4 channels interact with pyridine nucleotide-binding β-subunits that belong to the aldo-keto reductase superfamily. Binding of NADP(+) to Kvβ removes N-type inactivation of Kv currents, whereas NADPH stabilizes channel inactivation. Pyridine nucleotides also regulate Slo channels by interacting with their cytosolic regulator of potassium conductance domains that show high sequence homology to the bacterial TrkA family of K(+) transporters. These nucleotides also have been shown to modify the activity of the plasma membrane K(ATP) channels, the cystic fibrosis transmembrane conductance regulator, the transient receptor potential M2 channel, and the intracellular ryanodine receptor calcium release channels. In addition, pyridine nucleotides also modulate the voltage-gated sodium channel by supporting the activity of its ancillary subunit-the glycerol-3-phosphate dehydrogenase-like protein. Moreover, the NADP(+) metabolite, NAADP(+), regulates intracellular calcium homeostasis via the 2-pore channel, ryanodine receptor, or transient receptor potential M2 channels. Regulation of ion channels by pyridine nucleotides may be required for integrating cell ion transport to energetics and for sensing oxygen levels or metabolite availability. This mechanism also may be an important component of hypoxic pulmonary vasoconstriction, memory, and circadian rhythms, and disruption of this regulatory axis may be linked to dysregulation of calcium homeostasis and cardiac arrhythmias.

The diversity of microbial aldo/keto reductases from Escherichia coli K12

The genome of Escherichia coli K12 contains 9 open reading frames encoding aldo/keto reductases (AKRs) that are differentially regulated and sequence diverse. A significant amount of data is available for the E. coli AKRs through the availability of gene knockouts and gene expression studies, which adds to the biochemical and kinetic data. This together with the availability of crystal structures for nearly half of the E. coli AKRs and homologues of several others provides an opportunity to look at the diversity of these representative bacterial AKRs. Based around the common AKR fold of (β/α)8 barrel with two additional α-helices, the E. coli AKRs have a loop structure that is more diverse than their mammalian counterparts, creating a variety of active site architectures. Nearly half of the AKRs are expected to be monomeric, but there are examples of dimeric, trimeric and octameric enzymes, as well as diversity in specificity for NAD as well as NADP as a cofactor. However in functional assignments and characterisation of enzyme activities there is a paucity of data when compared to the mammalian AKR enzymes.

Potentiation of the Kv1 family K(+) channel by cortisone analogues

The Kv1 family voltage-dependent K(+) channels are essential for termination of action potentials in neurons and myocytes. These channels form a stable complex with their beta subunits (Kvβ), some of which inhibit channel activity. Cortisone potentiates Kv1 channel by binding to Kvβ and promoting its dissociation from the channel, but its half-maximum effective concentration is ∼46 μM. To identify corticosteroids that are more efficient than cortisone, we examined 25 cortisone analogues and found that fluticasone propionate potentiates channel current with a half-maximum effective concentration (EC(50)) of 37 ± 1.1 nM. Further studies showed that fluticasone propionate potentiates channel current by inducing dissociation of Kvβ, and docking of fluticasone propionate into the cortisone binding site reveals potential interactions that enhance the EC(50) value. Thus, fluticasone propionate provides a starting point for rational design of more efficient small-molecule compounds that increase Kv1 activity and affect the integrity of the Kv1-Kvβ complex.

Interactions between the C-terminus of Kv1.5 and Kvβ regulate pyridine nucleotide-dependent changes in channel gating

Voltage-gated potassium (Kv) channels are tetrameric assemblies of transmembrane Kv proteins with cytosolic N- and C-termini. The N-terminal domain of Kv1 proteins binds to β-subunits, but the role of the C-terminus is less clear. Therefore, we studied the role of the C-terminus in regulating Kv1.5 channel and its interactions with Kvβ-subunits. When expressed in COS-7 cells, deletion of the C-terminal domain of Kv1.5 did not affect channel gating or kinetics. Coexpression of Kv1.5 with Kvβ3 increased current inactivation, whereas Kvβ2 caused a hyperpolarizing shift in the voltage dependence of current activation. Inclusion of NADPH in the patch pipette solution accelerated the inactivation of Kv1.5-Kvβ3 currents. In contrast, NADP(+) decreased the rate and the extent of Kvβ3-induced inactivation and reversed the hyperpolarizing shift in the voltage dependence of activation induced by Kvβ2. Currents generated by Kv1.5ΔC+Kvβ3 or Kv1.5ΔC+Kvβ2 complexes did not respond to changes in intracellular pyridine nucleotide concentration, indicating that the C-terminus is required for pyridine nucleotide-dependent interactions between Kvβ and Kv1.5. A glutathione-S-transferase (GST) fusion protein containing the C-terminal peptide of Kv1.5 did not bind to apoKvβ2, but displayed higher affinity for Kvβ2:NADPH than Kvβ2:NADP(+). The GST fusion protein also precipitated Kvβ proteins from mouse brain lysates. Pull-down experiments, structural analysis and electrophysiological data indicated that a specific region of the C-terminus (Arg543-Val583) is required for Kvβ binding. These results suggest that the C-terminal domain of Kv1.5 interacts with β-subunits and that this interaction is essential for the differential regulation of Kv currents by oxidized and reduced nucleotides.

Oxidation of NADPH on Kvbeta1 inhibits ball-and-chain type inactivation by restraining the chain

The Kv1 family voltage-dependent K(+) channels assemble with cytosolic β subunits (Kvβ), which are composed of a flexible N terminus followed by a structured core domain. The N terminus of certain Kvβs inactivates the channel by blocking the ion conduction pore, and the core domain is a functional enzyme that uses NADPH as a cofactor. Oxidation of the Kvβ-bound NADPH inhibits inactivation and potentiates channel current, but the mechanism behind this effect is unknown. Here we show that after oxidation, the core domain binds to part of the N terminus, thus restraining it from blocking the channel. The interaction is partially mediated by two negatively charged residues on the core domain and three positively charged ones on the N terminus. These results provide a molecular basis for the coupling between the cellular redox state and channel activity, and establish Kvβ as a target for pharmacological control of Kv1 channels.

Deletion of the mouse homolog of KCNAB2, a gene linked to monosomy 1p36, results in associative memory impairments and amygdala hyperexcitability

Ablation of the distal end of the short arm of chromosome 1 [1p36 deletion syndrome (1p36DS)] is one of the most commonly occurring terminal deletion syndromes in humans, occurring in ∼1 in 5000 newborns. Subjects with 1p36DS manifest a wide range of clinical features including growth delay, congenital heart defects, and craniofacial dysmorphism. In addition, individuals with 1p36DS often exhibit some form of neurological abnormality and are typically cognitively impaired. Although there is significant variability with regard to the extent of the deletion, several genes have been mapped to region 1p36 that are known to regulate neuronal function. One such gene--KCNAB2--encodes the potassium channel auxiliary subunit Kvβ2, which has been previously shown to modulate voltage-gated potassium currents in heterologous expression systems. Here, we present experiments characterizing mice in which the ortholog of KCNAB2 was deleted. We find that deletion of Kcnab2 in mice leads to deficits in associative learning and memory. In addition, using whole-cell current-clamp, we find that deletion of Kcnab2 leads to a reduction in the slow afterhyperpolarization following a burst of action potentials and a concomitant increase in neuronal excitability in projection neurons in the lateral nucleus of the amygdala. Our results suggest that loss of Kvβ2 likely contributes to the cognitive and neurological impairments observed in 1p36DS patients.

Catalytic reduction of carbonyl groups in oxidized PAPC by Kvβ2 (AKR6)

The β-subunits of the voltage-gated potassium channel (Kvβ) belong to the aldo-keto reductase superfamily. The Kvβ-subunits dock with the pore-forming Kv α-subunits and impart or accelerate the rate of inactivation in Kv channels. Inactivation of Kv currents by Kvβ is differentially regulated by oxidized and reduced pyridine nucleotides. In mammals, AKR6 family is comprised of 3 different genes Kvβ1-3. We have shown previously that Kvβ2 catalyzes the reduction of a broad range of carbonyls including aromatic carbonyls, electrophilic aldehydes and prostaglandins. However, the endogenous substrates for Kvβ have not been identified. To determine whether products of lipid oxidation are substrates of Kvβs, we tested the enzymatic activity of Kvβ2 with oxidized phospholipids generated during the oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (PAPC). Electrospray ionization mass spectrometric analysis showed that Kvβ2 catalyzed the NADPH-dependent reduction of several products of oxPAPC, including 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphorylcholine (POVPC), 1-palmitoyl-2-(epoxycyclopentenone)-sn-glycero-3-phosphorylcholine (PECPC), 1-palmitoyl-2-(5,6)- epoxyisoprostane E2-sn-glycero-3-phosphocholine (PEIPC). These results were validated using high resolution mass spectrometric analysis. Time course analysis revealed that the reduced products reached significant levels for ions at m/z 594/596 (POVPC/PHVPC), 810/812 (PECPC/2H-PECPC) and 828/830 (PEIPC/2H-PEIPC) in the oxPAPC+Kvβ2 mixture (p<0.01). These results suggest that Kvβ could serve as a sensor of lipid oxidation via its catalytic activity and thereby alter Kv currents under conditions of oxidative stress.

Substrate profiling and aldehyde dismutase activity of the Kvβ2 subunit of the mammalian Kv1 potassium channel

Voltage-dependent potassium channels (Kv) are involved in various cellular signalling processes by governing the membrane potential of excitable cells. The cytosolic face of these α subunit-containing channels is associated with β subunits that can modulate channel responses. Surprisingly, the β subunit of the mammalian Kv1 channels, Kvβ2, has a high level of sequence homology with the aldo-keto reductase (AKR) superfamily of proteins. Recent studies have shown that Kvβ2 can catalyze the reduction of aldehydes and, most significantly, that channel function is modulated when Kvβ2-bound NADPH is concomitantly oxidized. As a result, the redox chemistry of this subunit is crucial to understanding its role in K(+) channel modulation. The present study has extended knowledge of the substrate profile of this subunit using a single turnover fluorimetric assay. Kvβ2 was found to catalyse the reduction of aromatic aldehyde substrates such as 2, 3 and 4-nitrobenzaldehydes, 4-hydroxybenzaldehyde, pyridine 2-aldehyde and benzaldehyde. The presence of an electron withdrawing group at the position para to the aldehyde in aromatic compounds facilitated reduction. Aliphatic aldehydes proved to be poor substrates. We devised a simple HPLC-based assay to identify Kvβ2 reaction products. Using this assay we showed, for the first time, that Kvβ2 can catalyze a slow aldehyde dismutation reaction using 4-nitrobenzaldehyde as substrate and have identified the products of this reaction. The ability of Kvβ2 to carry out both an aldehyde reduction and a dismutation reaction is discussed in the light of current thinking on the role of redox chemistry in channel modulation.

Activation of glucose-6-phosphate dehydrogenase promotes acute hypoxic pulmonary artery contraction

Hypoxic pulmonary vasoconstriction (HPV) is a physiological response to a decrease in airway O(2) tension, but the underlying mechanism is incompletely understood. We studied the contribution of glucose-6-phosphate dehydrogenase (Glc-6-PD), an important regulator of NADPH redox and production of reactive oxygen species, to the development of HPV. We found that hypoxia (95% N(2), 5% CO(2)) increased contraction of bovine pulmonary artery (PA) precontracted with KCl or serotonin. Depletion of extracellular glucose reduced NADPH, NADH, and HPV, substantiating the idea that glucose metabolism and Glc-6-PD play roles in the response of PA to hypoxia. Our data also show that inhibition of glycolysis and mitochondrial respiration (indicated by an increase in NAD(+) and decrease in the ATP-to-ADP ratio) by hypoxia, or by inhibitors of pyruvate dehydrogenase or electron transport chain complexes I or III, increased generation of reactive oxygen species, which in turn activated Glc-6-PD. Inhibition of Glc-6-PD decreased Ca(2+) sensitivity to the myofilaments and diminished Ca(2+)-independent and -dependent myosin light chain phosphorylation otherwise increased by hypoxia. Silencing Glc-6-PD expression in PA using a targeted small interfering RNA abolished HPV and decreased extracellular Ca(2+)-dependent PA contraction increased by hypoxia. Similarly, Glc-6-PD expression and activity were significantly reduced in lungs from Glc-6-PD(mut(-/-)) mice, and there was a corresponding reduction in HPV. Finally, regression analysis relating Glc-6-PD activity and the NADPH-to-NADP(+) ratio to the HPV response clearly indicated a positive linear relationship between Glc-6-PD activity and HPV. Based on these findings, we propose that Glc-6-PD and NADPH redox are crucially involved in the mechanism of HPV and, in turn, may play a key role in increasing pulmonary arterial pressure, which is involved in the development of pulmonary hypertension.

Coupling prokaryotic cell fate and division control with a bifunctional and oscillating oxidoreductase homolog

NAD(H)-binding proteins play important roles in cell-cycle and developmental signaling in eukaryotes. We identified a bifunctional NAD(H)-binding regulator (KidO) that integrates cell-fate signaling with cytokinesis in the bacterium Caulobacter crescentus. KidO stimulates the DivJ kinase and directly acts on the cytokinetic tubulin, FtsZ, to tune cytokinesis with the cell cycle. At the G1-->S transition, DivJ concomitantly signals the ClpXP-dependent degradation of KidO and CtrA, a cell-cycle transcriptional regulator/DNA replication inhibitor. This proteolytic event directs KidO and CtrA into oscillatory cell-cycle abundance patterns that coordinately license replication and cytokinesis. KidO resembles NAD(P)H-dependent oxidoreductases, and conserved residues in the KidO NAD(H)-binding pocket are critical for regulation of FtsZ, but not for DivJ. Since NADPH-dependent regulation by a KidO-like oxidoreductase also occurs in humans, organisms from two domains of life exploit the enzymatic fold of an ancestral oxidoreductase potentially to coordinate cellular or developmental activities with the availability of the metabolic currency, NAD(P)H.

Aldo-keto reductase (AKR) superfamily: genomics and annotation

Aldo-keto reductases (AKRs) are phase I metabolising enzymes that catalyse the reduced nicotinamide adenine dinucleotide (phosphate) (NAD(P)H)-dependent reduction of carbonyl groups to yield primary and secondary alcohols on a wide range of substrates, including aliphatic and aromatic aldehydes and ketones, ketoprostaglan-dins, ketosteroids and xenobiotics. In so doing they functionalise the carbonyl group for conjugation (phase II enzyme reactions). Although functionally diverse, AKRs form a protein superfamily based on their high sequence identity and common protein fold, the (α/(β)₈-barrel structure. Well over 150 AKR enzymes, from diverse organisms, have been annotated so far and given systematic names according to a nomenclature that is based on multiple protein sequence alignment and degree of identity. Annotation of non-vertebrate AKRs at the National Center for Biotechnology Information or Vertebrate Genome Annotation (vega) database does not often include the systematic nomenclature name, so the most comprehensive overview of all annotated AKRs is found on the AKR website (http://www.med.upenn.edu/akr/). This site also hosts links to more detailed and specialised information (eg on crystal structures, gene expression and single nucleotide polymorphisms [SNPs]). The protein-based AKR nomenclature allows unambiguous identification of a given enzyme but does not reflect the wealth of genomic and transcriptomic variation that exists in the various databases. In this context, identification of putative new AKRs and their distinction from pseudogenes are challenging. This review provides a short summary of the characteristic features of AKR biochemistry and structure that have been reviewed in great detail elsewhere, and focuses mainly on nomenclature and database entries of human AKRs that so far have not been subject to systematic annotation. Recent developments in the annotation of SNP and transcript variance in AKRs are also summarised.

Palmitate attenuates myocardial contractility through augmentation of repolarizing Kv currents

There is considerable evidence to support a role for lipotoxicity in the development of diabetic cardiomyopathy, although the molecular links between enhanced saturated fatty acid uptake/metabolism and impaired cardiac function are poorly understood. In the present study, the effects of acute exposure to the saturated fatty acid, palmitate, on myocardial contractility and excitability were examined directly. Exposure of isolated (adult mouse) ventricular myocytes to palmitate, complexed to bovine serum albumin (palmitate:BSA) as in blood, rapidly reduced (by 54+/-4%) mean (+/-SEM) unloaded fractional cell shortening. The amplitudes of intracellular Ca(2+) transients decreased in parallel. Current-clamp recordings revealed that exposure to palmitate:BSA markedly shortened action potential durations at 20%, 50%, and 90% repolarization. These effects were reversible and were occluded when the K(+) in the recording pipettes was replaced with Cs(+), suggesting a direct effect on repolarizing K(+) currents. Indeed, voltage-clamp recordings revealed that palmitate:BSA reversibly and selectively increased peak outward voltage-gated K(+) (Kv) current amplitudes by 20+/-2%, whereas inwardly rectifying K(+) (Kir) currents and voltage-gated Ca(2+) currents were unaffected. Further analyses revealed that the individual Kv current components I(to,f), I(K,slow) and I(ss), were all increased (by 12+/-2%, 37+/-4%, and 34+/-4%, respectively) in cells exposed to palmitate:BSA. Consistent with effects on both components of I(K,slow) (I(K,slow1) and I(K,slow)(2)) the magnitude of the palmitate-induced increase was attenuated in ventricular myocytes isolated from animals in which the Kv1.5 (I(K,slow)(1)) or the Kv2.1 (I(K,slow)(2)) locus was disrupted and I(K,slow)(1) or I(K,slow2) is eliminated. Both the enhancement of I(K,slow) and the negative inotropic effect of palmitate:BSA were reduced in the presence of the Kv1.5 selective channel blocker, diphenyl phosphine oxide-1 (DPO-1).Taken together, these results suggest that elevations in circulating saturated free fatty acids, as occurs in diabetes, can directly augment repolarizing myocardial Kv currents and impair excitation-contraction coupling.

The functional network of ion channels in T lymphocytes

For more than 25 years, it has been widely appreciated that Ca2+ influx is essential to trigger T-lymphocyte activation. Patch clamp analysis, molecular identification, and functional studies using blockers and genetic manipulation have shown that a unique contingent of ion channels orchestrates the initiation, intensity, and duration of the Ca2+ signal. Five distinct types of ion channels--Kv1.3, KCa3.1, Orai1+ stromal interacting molecule 1 (STIM1) [Ca2+-release activating Ca2+ (CRAC) channel], TRPM7, and Cl(swell)--comprise a network that performs functions vital for ongoing cellular homeostasis and for T-cell activation, offering potential targets for immunomodulation. Most recently, the roles of STIM1 and Orai1 have been revealed in triggering and forming the CRAC channel following T-cell receptor engagement. Kv1.3, KCa3.1, STIM1, and Orai1 have been found to cluster at the immunological synapse following contact with an antigen-presenting cell; we discuss how channels at the synapse might function to modulate local signaling. Immuno-imaging approaches are beginning to shed light on ion channel function in vivo. Importantly, the expression pattern of Ca2+ and K+ channels and hence the functional network can adapt depending upon the state of differentiation and activation, and this allows for different stages of an immune response to be targeted specifically.

The catalytic mechanism of NADH-dependent reduction of 9,10-phenanthrenequinone by Candida tenuis xylose reductase reveals plasticity in an aldo-keto reductase active site

Despite their widely varying physiological functions in carbonyl metabolism, AKR2B5 (Candida tenuis xylose reductase) and many related enzymes of the aldo-keto reductase protein superfamily utilise PQ (9,10-phenanthrenequinone) as a common in vitro substrate for NAD(P)H-dependent reduction. The catalytic roles of the conserved active-site residues (Tyr51, Lys80 and His113) of AKR2B5 in the conversion of the reactive alpha-dicarbonyl moiety of PQ are not well understood. Using wild-type and mutated (Tyr51, Lys80 and His113 individually replaced by alanine) forms of AKR2B5, we have conducted steady-state and transient kinetic studies of the effects of varied pH and deuterium isotopic substitutions in coenzyme and solvent on the enzymatic rates of PQ reduction. Each mutation caused a 10(3)-10(4)-fold decrease in the rate constant for hydride transfer from NADH to PQ, whose value in the wild-type enzyme was determined as approximately 8 x 10(2) s(-1). The data presented support an enzymic mechanism in which a catalytic proton bridge from the protonated side chain of Lys80 (pK=8.6+/-0.1) to the carbonyl group adjacent to the hydride acceptor carbonyl facilitates the chemical reaction step. His113 contributes to positioning of the PQ substrate for catalysis. Contrasting its role as catalytic general acid for conversion of the physiological substrate xylose, Tyr51 controls release of the hydroquinone product. The proposed chemistry of AKR2B5 action involves delivery of both hydrogens required for reduction of the alpha-dicarbonyl substrate to the carbonyl group undergoing (stereoselective) transformation. Hydride transfer from NADH probably precedes the transfer of a proton from Tyr51 whose pK of 7.3+/-0.3 in the NAD+-bound enzyme appears suitable for protonation of a hydroquinone anion (pK=8.8). These results show that the mechanism of AKR2B5 is unusually plastic in the exploitation of the active-site residues, for the catalytic assistance provided to carbonyl group reduction in alpha-dicarbonyls differs from that utilized in the conversion of xylose.

Aldo-keto reductases in which the conserved catalytic histidine is substituted

Aldo-keto reductases (AKRs) are a major superfamily of monomeric NADPH-dependent carbonyl oxidoreductases. They are characterized by an (alpha/beta)(8)-barrel structure, which at its base contains a conserved catalytic tetrad of Tyr, Lys, His and Asp. Two AKR subfamilies contain other residues substituted for the catalytic His and perform different functions. First, the steroid 5beta-reductase (AKR1D1), which reduces CC double bonds instead of carbonyl groups, has a Glu substituted for His. Second, the Kvbeta subunits (AKR6A3, AKR6A5 and AKR6A9) which modulate opening of the voltage-gated potassium channel (Kv1) by oxidizing NADPH, have an Asn substituted for the His. Previously, we noted that conserved catalytic residues in AKRs perform similar functions in the short-chain dehydrogenases (SDRs). With the availability of crystal structures of AKR1D1 and two SDRs that catalyze double-bond reduction reactions, Digitalis steroid 5beta-reductase and 2,4-dienoyl-CoA reductase, we have compared their active sites to outline the features that govern whether 1,2-, 1,4- or 1,6-hydride transfer occurs.

Kinetics of nucleotide binding to the beta-subunit (AKR6A2) of the voltage-gated potassium (Kv) channel

The beta-subunits of the voltage-gated potassium (Kv) channels modulate the kinetics and the gating of Kv channels and assists in channel trafficking and membrane localization. These proteins are members of the AKR6 family. They share a common (alpha/beta)(8) barrel structural fold and avidly bind pyridine nucleotides. Low catalytic activity has been reported for these proteins. Kinetic studies with rat Kvbeta2 revealed that the chemical step is largely responsible for the rate-limitation but nucleotide exchange could also contribute to the overall rate. Herein we report our investigations on the kinetics of cofactor exchange using nucleotide-free preparations of Kvbeta2. Kinetic traces measuring quenching of Kvbeta2 fluorescence by NADP(+) were consistent with a two-step binding mechanism which includes rapid formation of a loose enzyme:cofactor complex followed by a slow conformational rearrangement to form a tight final complex. Closing of the nucleotide enfolding loop, which in the crystal structure folds over the bound cofactor, provides the structural basis for this rearrangement. The rate of the loop opening required to release the cofactor is similar for NADPH and NADP(+) (0.9 min(-1)) and is of the same order of magnitude as the rate of the chemical step estimated previously from kinetic studies with 4-nitrobenzaldehyde (0.3-0.8 min(-1), [S.M. Tipparaju, O.A. Barski, S. Srivastava, A. Bhatnagar, Catalytic mechanism and substrate specificity of the beta-subunit of the voltage-gated potassium channel, Biochemistry 47 (2008) 8840-8854]). Binding of NADPH is accompanied by a second conformational change that might be responsible for a 4-fold higher affinity observed with the reduced cofactor and the resulting difficulty in removing bound NADPH from the protein. These data provide evidence that nucleotide exchange occurs on a seconds-to-minutes time scale and set the upper limit for the maximal possible rate of catalysis by Kvbeta2. Slow cofactor exchange is consistent with the role of the beta-subunit as a metabolic sensor implicated in tonic regulation of potassium currents.

The aldo-keto reductase superfamily and its role in drug metabolism and detoxification

The aldo-keto reductase (AKR) superfamily comprises enzymes that catalyze redox transformations involved in biosynthesis, intermediary metabolism, and detoxification. Substrates of AKRs include glucose, steroids, glycosylation end-products, lipid peroxidation products, and environmental pollutants. These proteins adopt a (beta/alpha)(8) barrel structural motif interrupted by a number of extraneous loops and helixes that vary between proteins and bring structural identity to individual families. The human AKR family differs from the rodent families. Due to their broad substrate specificity, AKRs play an important role in the phase II detoxification of a large number of pharmaceuticals, drugs, and xenobiotics.