myo-Inositol oxygenase from hog kidney. I. Purification and characterization of the oxygenase and of an enzyme complex containing the oxygenase and D-glucuronate reductase

Characterization of myo-inositol oxygenase from rice (OsMIOX): influence of salinity stress in different indica rice cultivars

Myo-inositol oxygenase (MIOX), the only catabolic enzyme of the inositol pathway, catalyzes conversion of myo-inositol to D-GlcA (glucuronic acid). The present study encompasses bioinformatic analysis of MIOX gene across phylogenetically related plant lineages and representative animal groups. Comparative motif analysis of the MIOX gene(s) across various plant groups suggested existence of abiotic- stress related cis-acting elements such as, DRE, MYB, MYC, STRE, MeJa among others. A detailed analysis revealed a single isoform of MIOX gene, located in chromosome 6 of indica rice (Oryza sativa) with an open reading frame of 938 bp coding for 308 amino acids producing a protein of ~ 35 kD. Secondary structure prediction of the protein gave the predicted number of 144 alpha helices and 154 random coils. The three-dimensional structure suggested it to be a monomeric protein with a single domain. Bacterial overexpression of the protein, purification and enzyme assay showed optimal catalytic activity at pH 7.5-8 at an optimal temperature of 37 °C with Michaelis constant of 40.92 mM. The range of Km was determined as 22.74-28.7 mM and the range of Vmax was calculated as 3.51-3.6 µM/min, respectively. Four salt-tolerant and salt-sensitive rice cultivars displayed differential gene expression of OsMIOX at different time points in different tissues under salinity and drought stress as observed from qRT-PCR data, microarray results and protein expression profile in immunoblot analysis. Gel volumetric analysis confirmed a very high expression of MIOX in roots and leaves on 7th day following germination. Microarray data showed high expression of MIOX at all developmental stages including seedling growth and reproduction. These data suggest that OsMIOX might have a role to play in rice abiotic stress responses mediated through the myo-inositol oxidation pathway.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01340-6.

Inositol in Disease and Development: Roles of Catabolism via myo-Inositol Oxygenase in Drosophila melanogaster

Inositol depletion has been associated with diabetes and related complications. Increased inositol catabolism, via myo-inositol oxygenase (MIOX), has been implicated in decreased renal function. This study demonstrates that the fruit fly Drosophila melanogaster catabolizes myo-inositol via MIOX. The levels of mRNA encoding MIOX and MIOX specific activity are increased when fruit flies are grown on a diet with inositol as the sole sugar. Inositol as the sole dietary sugar can support D. melanogaster survival, indicating that there is sufficient catabolism for basic energy requirements, allowing for adaptation to various environments. The elimination of MIOX activity, via a piggyBac WH-element inserted into the MIOX gene, results in developmental defects including pupal lethality and pharate flies without proboscises. In contrast, RNAi strains with reduced levels of mRNA encoding MIOX and reduced MIOX specific activity develop to become phenotypically wild-type-appearing adult flies. myo-Inositol levels in larval tissues are highest in the strain with this most extreme loss of myo-inositol catabolism. Larval tissues from the RNAi strains have inositol levels higher than wild-type larval tissues but lower levels than the piggyBac WH-element insertion strain. myo-Inositol supplementation of the diet further increases the myo-inositol levels in the larval tissues of all the strains, without any noticeable effects on development. Obesity and blood (hemolymph) glucose, two hallmarks of diabetes, were reduced in the RNAi strains and further reduced in the piggyBac WH-element insertion strain. Collectively, these data suggest that moderately increased myo-inositol levels do not cause developmental defects and directly correspond to reduced larval obesity and blood (hemolymph) glucose.

Regulations of myo-inositol homeostasis: Mechanisms, implications, and perspectives

Phosphorylation is the most common module of cellular signalling pathways. The dynamic nature of phosphorylation, which is conferred by the balancing acts of kinases and phosphatases, allows this modification to finely control crucial cellular events such as growth, differentiation, and cell cycle progression. Although most research to date has focussed on protein phosphorylation, non-protein phosphorylation substrates also play vital roles in signal transduction. The most well-established substrate of non-protein phosphorylation is inositol, whose phosphorylation generates many important signalling molecules such as the second messenger IP₃, a key factor in calcium signalling. A fundamental question to our understanding of inositol phosphorylation is how the levels of cellular inositol are controlled. While the availability of protein phosphorylation substrates is known to be readily controlled at the levels of transcription, translation, and/or protein degradation, the regulatory mechanisms that control the uptake, synthesis, and removal of inositol are underexplored. Potentially, such mechanisms serve as an important layer of regulation of cellular signal transduction pathways. There are two ways in which mammalian cells acquire inositol. The historic use of radioactive ³H-myo-inositol revealed that inositol is promptly imported from the extracellular environment by three specific symporters SMIT1/2, and HMIT, coupling sodium or proton entry, respectively. Inositol can also be synthesized de novo from glucose-6P, thanks to the enzymatic activity of ISYNA1. Intriguingly, emerging evidence suggests that in mammalian cells, de novo myo-inositol synthesis occurs irrespective of inositol availability in the environment, prompting the question of whether the two sources of inositol go through independent metabolic pathways, thus serving distinct functions. Furthermore, the metabolic stability of myo-inositol, coupled with the uptake and endogenous synthesis, determines that there must be exit pathways to remove this extraordinary sugar from the cells to maintain its homeostasis. This essay aims to review our current knowledge of myo-inositol homeostatic metabolism, since they are critical to the signalling events played by its phosphorylated forms.

Metabolic engineering of Saccharomyces cerevisiae for efficient production of glucaric acid at high titer

BACKGROUND

Glucaric acid is a high-value-added chemical that can be used in various fields. Because chemical oxidation of glucose to produce glucaric acid is not environmentally friendly, microbial production has attracted increasing interest recently. Biological pathways to synthesize glucaric acid from glucose in both Escherichia coli and Saccharomyces cerevisiae by co-expression of genes encoding myo-inositol-1-phosphate synthase (Ino1), myo-inositol oxygenase (MIOX), and uronate dehydrogenase (Udh) have been constructed. However, low activity and instability of MIOX from Mus musculus was proved to be the bottleneck in this pathway.

RESULTS

A more stable miox4 from Arabidopsis thaliana was chosen in the present study. In addition, high copy delta-sequence integration of miox4 into the S. cerevisiae genome was performed to increase its expression level further. Enzymatic assay and quantitative real-time PCR analysis revealed that delta-sequence-based integrative expression increased MIOX4 activity and stability, thus increasing glucaric acid titer about eight times over that of episomal expression. By fed-batch fermentation supplemented with 60 mM (10.8 g/L) inositol, the multi-copy integrative expression S. cerevisiae strain produced 6 g/L (28.6 mM) glucaric acid from myo-inositol, the highest titer that had been ever reported in S. cerevisiae.

CONCLUSIONS

In this study, glucaric acid titer was increased to 6 g/L in S. cerevisiae by integrating the miox4 gene from A. thaliana and the udh gene from Pseudomonas syringae into the delta sequence of genomes. Delta-sequence-based integrative expression increased both the number of target gene copies and their stabilities. This approach could be used for a wide range of metabolic pathway engineering applications with S. cerevisiae.

myo-Inositol Oxygenase Overexpression Accentuates Generation of Reactive Oxygen Species and Exacerbates Cellular Injury following High Glucose Ambience: A NEW MECHANISM RELEVANT TO THE PATHOGENESIS OF DIABETIC NEPHROPATHY

Diabetic nephropathy (DN) is characterized by perturbations in metabolic/cellular signaling pathways with generation of reactive oxygen species (ROS). The ROS are regarded as a common denominator of various pathways, and they inflict injury on renal glomerular cells. Recent studies indicate that tubular pathobiology also plays a role in the progression of DN. However, the mechanism(s) for how high (25 mm) glucose (HG) ambience induces tubular damage remains enigmatic. myo-Inositol oxygenase (MIOX) is a tubular enzyme that catabolizes myo-inositol to d-glucuronate via the glucuronate-xylulose (G-X) pathway. In this study, we demonstrated that G-X pathway enzymes are expressed in the kidney, and MIOX expression/bioactivity was up-regulated under HG ambience in LLC-PK1 cells, a tubular cell line. We further investigated whether MIOX overexpression leads to accentuation of tubulo-interstitial injury, as gauged by some of the parameters relevant to the progression of DN. Under HG ambience, MIOX overexpression accentuated redox imbalance, perturbed NAD(+)/NADH ratios, increased ROS generation, depleted reduced glutathione, reduced GSH/GSSG ratio, and enhanced adaptive changes in the profile of the antioxidant defense system. These changes were also accompanied by mitochondrial dysfunctions, DNA damage and induction of apoptosis, accentuated activity of profibrogenic cytokine, and expression of fibronectin, the latter two being the major hallmarks of DN. These perturbations were largely blocked by various ROS inhibitors (Mito Q, diphenyleneiodonium chloride, and N-acetylcysteine) and MIOX/NOX4 siRNA. In conclusion, this study highlights a novel mechanism where MIOX under HG ambience exacerbates renal injury during the progression of diabetic nephropathy following the generation of excessive ROS via an unexplored G-X pathway.

Renal depletion of myo-inositol is associated with its increased degradation in animal models of metabolic disease

Renal depletion of myo-inositol (MI) is associated with the pathogenesis of diabetic nephropathy in animal models, but the underlying mechanisms involved are unclear. We hypothesized that MI depletion was due to changes in inositol metabolism and therefore examined the expression of genes regulating de novo biosynthesis, reabsorption, and catabolism of MI. We also extended the analyses from diabetes mellitus to animal models of dietary-induced obesity and hypertension. We found that renal MI depletion was pervasive across these three distinct disease states in the relative order: hypertension (−51%) > diabetes mellitus (−35%) > dietary-induced obesity (−19%). In 4-wk diabetic kidneys and in kidneys derived from insulin-resistant and hypertensive rats, MI depletion was correlated with activity of the MI-degrading enzyme myo-inositol oxygenase (MIOX). By contrast, there was decreased MIOX expression in 8-wk diabetic kidneys. Immunohistochemistry localized the MI-degrading pathway comprising MIOX and the glucuronate-xylulose (GX) pathway to the proximal tubules within the renal cortex. These findings indicate that MI depletion could reflect increased catabolism through MIOX and the GX pathway and implicate a common pathological mechanism contributing to renal oxidative stress in metabolic disease.

Manipulation of L-ascorbic acid biosynthesis pathways in Solanum lycopersicum: elevated GDP-mannose pyrophosphorylase activity enhances L-ascorbate levels in red fruit

Ascorbate (AsA) plays a fundamental role in redox homeostasis in plants and animals, primarily by scavenging reactive oxygen species. Three genes, representing diverse steps putatively involved in plant AsA biosynthesis pathways, were cloned and independently expressed in Solanum lycopersicum (tomato) under the control of the CaMV 35S promoter. Yeast-derived GDP-mannose pyrophosphorylase (GMPase) and arabinono-1,4-lactone oxidase (ALO), as well as myo-inositol oxygenase 2 (MIOX2) from Arabidopsis thaliana, were targeted. Increases in GMPase activity were concomitant with increased AsA levels of up to 70% in leaves, 50% in green fruit, and 35% in red fruit. Expression of ALO significantly pulled biosynthetic flux towards AsA in leaves and green fruit by up to 54 and 25%, respectively. Changes in AsA content in plants transcribing the MIOX2 gene were inconsistent in different tissue. On the other hand, MIOX activity was strongly correlated with cell wall uronic acid levels, suggesting that MIOX may be a useful tool for the manipulation of cell wall composition. In conclusion, the Smirnoff-Wheeler pathway showed great promise as a target for biotechnological manipulation of ascorbate levels in tomato.

Insights into the (superoxo)Fe(III)Fe(III) intermediate and reaction mechanism of myo-inositol oxygenase: DFT and ONIOM(DFT:MM) study

The (superoxo)Fe(III)Fe(III) reactive species and the catalytic reaction mechanism of a diiron enzyme, myo-inositol oxygenase (MIOX), were theoretically investigated by means of density functional theory (DFT) and ONIOM quantum mechanical/molecular mechanical (QM/MM) approaches. The ground state of the (superoxo)Fe(III)Fe(III) intermediate was shown to have a side-on coordination geometry and an S = 1/2 spin state, wherein the two iron sites are antiferromagnetically coupled while the superoxide site and the nearest iron are ferromagnetically coupled. A full reaction pathway leading to a D-glucuronate product from myo-inositol was proposed based on ONIOM computational results. Two major roles of the enzyme surrounding during the catalytic reaction were identified. One is to facilitate the initial H-abstraction step, and the other is to restrict the movement of the substrate via H-bonding interactions in order to avoid unwanted side reactions. In our proposed mechanism, O-O bond cleavage has the highest barrier, thus constituting the rate-limiting step of the reaction. The unique role of the bridging hydroxide ligand as a catalytic base was also identified.

myo-Inositol oxygenase: a radical new pathway for O(2) and C-H activation at a nonheme diiron cluster

The enzyme myo-inositol oxygenase (MIOX) catalyzes conversion of myo-inositol (cyclohexan-1,2,3,5/4,6-hexa-ol or MI) to d-glucuronate (DG), initiating the only known pathway in humans for catabolism of the carbon skeleton of cell-signaling inositol (poly)phosphates and phosphoinositides. Recent kinetic, spectroscopic and crystallographic studies have shown that the enzyme activates its substrates, MI and O(2), at a carboxylate-bridged nonheme diiron(ii/iii) cluster, making it the first of many known nonheme diiron oxygenases to employ the mixed-valent form of its cofactor. Evidence suggests that: (1) the Fe(iii) site coordinates MI via its C1 and C6 hydroxyl groups; (2) the Fe(ii) site reversibly coordinates O(2) to produce a superoxo-diiron(iii/iii) intermediate; and (3) the pendant oxygen atom of the superoxide ligand abstracts hydrogen from C1 to initiate the unique C-C-bond-cleaving, four-electron oxidation reaction. This review recounts the studies leading to the recognition of the novel cofactor requirement and catalytic mechanism of MIOX and forecasts how remaining gaps in our understanding might be filled by additional experiments.

Production of glucaric acid from a synthetic pathway in recombinant Escherichia coli

A synthetic pathway has been constructed for the production of glucuronic and glucaric acids from glucose in Escherichia coli. Coexpression of the genes encoding myo-inositol-1-phosphate synthase (Ino1) from Saccharomyces cerevisiae and myo-inositol oxygenase (MIOX) from mice led to production of glucuronic acid through the intermediate myo-inositol. Glucuronic acid concentrations up to 0.3 g/liter were measured in the culture broth. The activity of MIOX was rate limiting, resulting in the accumulation of both myo-inositol and glucuronic acid as final products, in approximately equal concentrations. Inclusion of a third enzyme, uronate dehydrogenase (Udh) from Pseudomonas syringae, facilitated the conversion of glucuronic acid to glucaric acid. The activity of this recombinant enzyme was more than 2 orders of magnitude higher than that of Ino1 and MIOX and increased overall flux through the pathway such that glucaric acid concentrations in excess of 1 g/liter were observed. This represents a novel microbial system for the biological production of glucaric acid, a "top value-added chemical" from biomass.

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.

Crystal structure of a substrate complex of myo-inositol oxygenase, a di-iron oxygenase with a key role in inositol metabolism

Altered metabolism of the inositol sugars myo-inositol (MI) and d-chiro-inositol is implicated in diabetic complications. In animals, catabolism of MI and D-chiro-inositol depends on the enzyme MI oxygenase (MIOX), which catalyzes the first committed step of the glucuronate-xylulose pathway, and is found almost exclusively in the kidneys. The crystal structure of MIOX, in complex with MI, has been determined by multiwavelength anomalous diffraction methods and refined at 2.0-A resolution (R=0.206, Rfree=0.253). The structure reveals a monomeric, single-domain protein with a mostly helical fold that is distantly related to the diverse HD domain superfamily. Five helices form the structural core and provide six ligands (four His and two Asp) for the di-iron center, in which the two iron atoms are bridged by a putative hydroxide ion and one of the Asp ligands, Asp-124. A key loop forms a lid over the MI substrate, which is coordinated in bidentate mode to one iron atom. It is proposed that this mode of iron coordination, and interaction with a key Lys residue, activate MI for bond cleavage. The structure also reveals the basis of substrate specificity and suggests routes for the development of specific MIOX inhibitors.

Evidence for C-H cleavage by an iron-superoxide complex in the glycol cleavage reaction catalyzed by myo-inositol oxygenase

myo-Inositol oxygenase (MIOX) activates O2 at a mixed-valent nonheme diiron(II/III) cluster to effect oxidation of its cyclohexan-(1,2,3,4,5,6-hexa)-ol substrate [myo-inositol (MI)] by four electrons to d-glucuronate. Abstraction of hydrogen from C1 by a formally (superoxo)diiron(III/III) intermediate was previously proposed. Use of deuterium-labeled substrate, 1,2,3,4,5,6-[2H]6-MI (D6-MI), has now permitted initial characterization of the C-H-cleaving intermediate. The MIOX.1,2,3,4,5,6-[2H]6-MI complex reacts rapidly and reversibly with O2 to form an intermediate, G, with a g = (2.05, 1.98, 1.90) EPR signal. The rhombic g-tensor and observed hyperfine coupling to 57Fe are rationalized in terms of a (superoxo)diiron(III/III) structure with coordination of the superoxide to a single iron. G decays to H, the intermediate previously detected in the reaction with unlabeled substrate. This step is associated with a kinetic isotope effect of > or =5, showing that the superoxide-level complex does indeed cleave a C-H(D) bond of MI.

Expression of myo-inositol oxygenase in tissues susceptible to diabetic complications

Alterations of intracellular levels of myo-inositol (MI) have the potential to impact such cellular processes as signaling pathways and osmotic balance. Depletion of MI has been implicated in the etiology of diabetic complications; however, the mechanistic details remain sketchy. myo-Inositol oxygenase (MIOX-EC 1.13.99.1) catalyzes the first committed step of the only pathway of MI catabolism. In the present study, extra-renal tissues and cell types, including those affected by diabetic complications, were examined for MIOX expression. Western blotting results indicated that kidney is the only major organ where MIOX protein is expressed at detectable levels. Immunohistochemical examination of the kidney revealed that the proximal tubular epithelial cells are the only site of MIOX expression in the kidney. Reverse-transcription-polymerase chain reaction (RT-PCR) and Western immunoblot analyses, however, revealed that the cell lines ARPE-19 and HLE-B3, representing human retinal pigmented epithelium and lens epithelium, respectively, also express MIOX. In addition, quantitative real-time RT-PCR analysis of all major tissues in the mouse showed that the sciatic nerve contained MIOX transcript, which was found to be significantly higher than that observed in other non-renal organs. These results indicate that MIOX is found at lower levels in extra-renal tissues where diabetic complications, including nephropathy, neuropathy, retinopathy, and cataract, are frequently observed.

Developmental expression and function of aldehyde reductase in proximal tubules of the kidney

Aldehyde reductase reduces a wide variety of toxic and physiological aldehydes with a marked preference for negatively charged substrates such as glucuronate. Reduction of glucuronate to gulonate is a step in inositol catabolism, a process specific to the kidney cortex. Administration of the aldehyde reductase inhibitor AL-1576 to mice increases urinary output of glucuronate and decreases output of vitamin C. Aldehyde reductase mRNA with a 319-bp 5′-untranslated region is expressed ubiquitously in murine tissues. A new isoform with a short 64-bp 5′-untranslated region is found predominantly in the kidney, resulting in 10-fold higher enzymatic activity observed in this organ compared with other tissues. A moderate level of the new transcript is found in liver, intestine, and stomach, whereas brain, heart, lung, spleen, ovary, and testis have low to insignificant levels. The short transcript is absent during embryonic development and is first observed in the murine kidney on postnatal day 6. The abundance of the short transcript and enzyme activity increase sigmoidally with age; the sharpest increase occurs during the third week of life. As shown by immunohistochemistry, aldehyde reductase expression is limited to the proximal tubules and parietal epithelium of Bowman’s capsule. In the mouse, the intensity of staining in tubules increases with age, suggesting that induction of aldehyde reductase expression is part of renal tubular maturation. The human kidney also exhibits proximal tubular localization and the two mRNA transcripts of aldehyde reductase. Immunoreactive protein is present in the 9-wk-old fetal kidney, indicating that the induction of aldehyde reductase in humans occurs early in development.

The inositol oxygenase gene family of Arabidopsis is involved in the biosynthesis of nucleotide sugar precursors for cell-wall matrix polysaccharides

The nucleotide sugar UDP-glucuronic acid (UDP-GlcA) is the principal precursor for galacturonic acid, xylose, apiose and arabinose residues of the plant cell-wall polymers. UDP-GlcA can be synthesized by two different functional pathways in Arabidopsis involving either UDP-glucose dehydrogenase or inositol oxygenase as the initial enzyme reaction to channel carbohydrates into a pool of UDP sugars used for cell-wall biosynthesis. The genes for the enzyme myo-inositol oxygenase (MIOX) were analyzed in Arabidopsis. They represent a small gene family containing four members. The transcription of all those members indicates a transient and organ-specific gene expression pattern in growing plant tissues as analyzed by RT-PCR and in promoter::GUS reporter gene lines. Two isoforms (MIOX1, MIOX2) are expressed in almost all tissues of the plant, whereas the expression of MIOX4 and MIOX5 is largely restricted to flowers, particularly maturing pollen. T-DNA insertion lines in MIOX genes were isolated; however, single knock-outs show growth phenotypes similar to the wild type. The monosaccharide composition of the cell wall in these mutants is not significantly changed compared to wild type plants. However, the incorporation of 3H-inositol into wall polymers of seedlings is greatly impaired in the mutant lines (Delta)MIOX1 and (Delta)MIOX2, which are the only isoforms that are expressed in seedlings.

Up-regulation of Human myo-Inositol Oxygenase by Hyperosmotic Stress in Renal Proximal Tubular Epithelial Cells

myo-Inositol oxygenase (MIOX) catalyzes the oxidative cleavage of myo-inositol (MI) to give d-glucuronic acid, a committed step in MI catabolism. d-Glucuronic acid is further metabolized to xylitol via the glucuronate-xylulose pathway. Although accumulation of polyols such as xylitol and sorbitol is associated with MI depletion in diabetic complications, no causal relationship has been established. Therefore we are examining the role of MIOX in diabetic nephropathy. Here we present evidence that the basis for the depletion of MI in diabetes is likely to be mediated by the increased expression of MIOX, which is induced by sorbitol, mannitol, and xylitol in a porcine renal proximal tubular epithelial cell line, LLC-PK1. To understand the molecular mechanism of regulation of MIOX expression by polyols, we have cloned the human MIOX gene locus of 10 kb containing 5.6 kb of the 5' upstream sequence. Analysis of the 5' upstream sequence led to the identification of an osmotic response element (ORE) in the promoter region, which is present approximately 2 kb upstream of the translation start site. Based on luciferase reporter and electrophoretic mobility shift assays, polyols increased the ORE-dependent expression of MIOX. In addition, we demonstrate that the activity of the promoter is dependent on the binding of the transcription factor, tonicity element-binding protein, or osmotic response element-binding protein, to the ORE site. These results suggest that the expression of MIOX is up-regulated by a positive feedback mechanism where xylitol, one of the products of MI catabolism via the glucuronate-xylulose pathway, induces an overexpression of MIOX.

Molecular cloning, expression, and characterization of myo-inositol oxygenase from mouse, rat, and human kidney

myo-Inositol oxygenase (MIOX) is a non-heme iron enzyme, which catalyzes the conversion of myo-inositol to d-glucuronic acid, the first committed step in myo-inositol catabolism. Full-length cDNAs of 858bp each coding for 33kDa protein were cloned from kidney cDNA libraries of mouse, rat, and human. The individual clones were expressed in Escherichia coli and recombinant MIOX proteins were purified to electrophoretic homogeneity. A hydrophobic interaction chromatography step yielded multiple conformers, with mouse and human MIOX showing three peaks and rat enzyme revealing two peaks. Individual MIOX peaks exhibited distinct V(max) and K(m) values. Interestingly, upon storage, the 33kDa protein was degraded to a approximately 30kDa truncated protein in each species, and formed small amounts of dimers of identical subunits. While MIOX is a highly conserved enzyme in all mammalian species, the labile nature and tendency to degrade in solution may be the source of significant differences in size previously reported in the literature. Regardless of the source, our results strongly dispel previous conflicting literature reports on the size of the protein and confirm that MIOX is a 33kDa protein.

UDP-glucose dehydrogenase plays multiple roles in the biology of the pathogenic fungus Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic fungus surrounded by an elaborate polysaccharide capsule that is strictly required for its virulence in humans and other mammals. Nearly half of the sugar residues in the capsule are derived from UDP-glucuronic acid or its metabolites. To examine the role of these nucleotide sugars in C. neoformans, the gene encoding UDP-glucose dehydrogenase was disrupted. Mass spectrometry analysis of nucleotide sugar pools showed that the resulting mutant lacked both UDP-glucuronic acid and its downstream product, UDP-xylose, thus confirming the effect of the knockout and indicating that an alternate pathway for UDP-glucuronic acid production was not used. The mutant was dramatically affected by the lack of specific sugar donors, demonstrating altered cell integrity, temperature sensitivity, lack of growth in an animal model of cryptococcosis, and morphological defects. Additionally, the polysaccharide capsule could not be detected on the mutant cells, although the possibility remains that abbreviated forms of capsule components are made, possibly without proper surface display. The capsule defect is largely independent of the other observed changes, as cells that are acapsular because of mutations in other genes show lack of virulence but do not exhibit alterations in cell integrity, temperature sensitivity, or cellular morphology. All of the observed alterations were reversed by correction of the gene disruption.

myo-inositol oxygenase offers a possible entry point into plant ascorbate biosynthesis

Two biosynthetic pathways for ascorbate (l-ascorbic acid [AsA]; vitamin C) in plants are presently known, the mannose/l-galactose pathway and an l-GalUA pathway. Here, we present molecular and biochemical evidence for a possible biosynthetic route using myo-inositol (MI) as the initial substrate. A MI oxygenase (MIOX) gene was identified in chromosome 4 (miox4) of Arabidopsis ecotype Columbia, and its enzymatic activity was confirmed in bacterially expressed recombinant protein. Miox4 was primarily expressed in flowers and leaves of wild-type Arabidopsis plants, tissues with a high concentration of AsA. Ascorbate levels increased 2- to 3-fold in homozygous Arabidopsis lines overexpressing the miox4 open reading frame, thus suggesting the role of MI in AsA biosynthesis and the potential for using this gene for the agronomic and nutritional enhancement of crops.

Purification, characterization and functional cloning of inositol oxygenase from Cryptococcus

The enzyme inositol oxygenase (myo-inositol : oxygen oxidoreductase; E.C. 1.13.99.1) is a monooxygenase that converts inositol into glucuronic acid in the presence of molecular oxygen. This enzyme is integrated into a pathway leading to either degradation and energy production or the biosynthesis of precursors for polysaccharides. The enzyme was purified from the yeast Cryptococcus lactativorus by a five-step chromatography procedure. The purified enzyme shows a molecular mass of 37 kDa on SDS-PAGE, similar to the estimation of the size of the native enzyme determined by size exclusion chromatography. Peptides of the inositol oxygenase protein derived from a tryptic digest were sequenced de novo by nanoelectrospray tandem mass spectrometry. Using degenerate oligonucleotides, the corresponding gene was cloned from first strand cDNA. The open reading frame encodes a 315 amino acid polypeptide with a predicted molecular mass of 36.9 kDa. Inositol oxygenase is a single copy gene in C. lactativorus. It has close homologues in other fungi such as Cryptococcus neoformans and Neurospora crassa. Biochemical characterization of the enzyme showed a pH optimum of 6-6.5 and a temperature optimum of 30 degrees C. Myo-inositol is the only accepted substrate with a Km of ca. 5 mM. The enzyme contains a Fe-centre but the enzyme activity is resistant to KCN.

Aberrant 3H in Ehrlich mouse ascites tumor cell nucleotides after in vivo labeling with myo-[2-3H]- and L-myo-[1-3H]inositol: implications for measuring inositol phosphate signaling

After in vivo radiolabeling of Ehrlich cells for 24h with conventional myo-[2-3H]inositol we previously demonstrated an aberrant 3H-labeling of ATP that interfered in the HPLC analysis of inositol trisphosphates. This aberrant 3H-labeling was accounted for by the extensive kidney catabolism of myo-[2-3H] inositol with delivery of 3H-labeled metabolites to extrarenal tissues. As expected, the aberrant labeling of ATP is markedly reduced with the use of 3H-myo-inositol labeled at L-C1 rather than at C2, reflecting that the 3H at L-C1 disappears in the first step of the myo-inositol catabolism: the oxidative conversion to D-glucuronate. In contrast, with the 3H at C2 of myo-inositol, the 3H-C2 passes into the pentose phosphate conversions with resulting labeling of nucleotides. The extent of catabolism to 3H-labeled water, the cellular accumulation of 3H-myo-inositol, the incorporation into cellular inositol phospholipids, and the labeling pattern of cellular phosphoinositides were all found to be similar for the two labeled myo-inositol moieties. With the use of L-myo-[1-3H]inositol an aberrant 3H-labeling at about 25% remained, for which a presumptive mechanism is proposed. L-myo-[1-3H]Inositol appears nevertheless to be a preferable alternative to myo-[2-3H]inositol for tracing the intact myo-inositol molecule after in vivo labeling, with minimized interference from aberrant 3H-labeling of nucleotides.

Inositol synthesis and catabolism in Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that synthesizes and catabolizes inositol. This study demonstrates inositol synthesis from glucose-6-phosphate via inositol-1-phosphate synthase and catabolism to glucuronic acid via inositol oxygenase in this organism. These inositol synthetic and catabolic pathways are regulated in opposition; repressing conditions for one are inducing conditions for the other. An inositol-requiring strain was generated by UV mutagenesis. Without inositol, this mutant strain undergoes 'inositol-less' death, during which time the phosphatidylinositol composition of the membranes decreases without alteration of the proportion of other phospholipids. The mutation on this strain results in no detectable inositol synthetic activity but normal (wild-type) inositol catabolic activity. This inositol-requiring mutant strain reverted at a high frequency. Classical genetic experiments revealed that the majority of the reverting mutations are at second sites. Interestingly, the revertants exhibited unusual morphological phenotypes when deprived of inositol, while provision of inositol restored wild-type morphology. Inositol metabolism is clearly important for growth and development of C. neoformans and may be involved in this organism's mechanism for survival as both a saprophyte in soil and a parasite in humans.

An overview of gamma-hydroxybutyrate catabolism: the role of the cytosolic NADP(+)-dependent oxidoreductase EC 1.1.1.19 and of a mitochondrial hydroxyacid-oxoacid transhydrogenase in the initial, rate-limiting step in this pathway

Two enzymes have been found which catalyze the initial step in the catabolism of GHB. The oxidation of GHB to SSA, catalyzed by both of these enzymes, is coupled to the reduction of an oxoacid. In the case of the mitochondrial transhydrogenase, the coupling is obligatory. Although coupling is not obligatory for the GHB dehydrogenase, the stimulation provided by the coupled reaction, and the nature of the kinetics of the uncoupled reaction, may not only allow the reaction to proceed, but may provide a means of regulating the rate of the reaction under in vivo conditions. Since the oxidation of GHB to SSA is the rate limiting step in the overall catabolic pathway (the rate of conversion of GHB to SSA proceeds at approximately one one thousandth of the rate at which SSA is oxidized to succinate by SSA dehydrogenase (30)), factors which regulate the rate of either or both of these enzymes will, in turn, influence tissue levels of endogenous GHB as well as the duration and magnitude of the physiological effect of a dose of GHB.

Enhancement of aldose reductase activity by modification of an active site lysine: a possible mechanism for in vivo activation

Reaction of aldose reductase (ALR2) from pig muscle with pyridoxal 5'-phosphate (pyridoxal-P) and other lysine modifying reagents resulted in activation of the enzyme. The activation by pyridoxal-P showed pH and concentration dependence that was prevented by incubation with NADPH and various cofactor analogues but not by the aldehyde substrate. Spectral analysis of the reaction showed characteristic peaks associated with Schiff's base formation between a lysine amino group and the aldehyde of pyridoxal-P. Subsequent reduction produced spectra characteristic of a phosphopyridoxyllysine bond. Phosphopyridoxyllysine was isolated by amino acid analysis of modified ALR2. Determination of the stoichiometry of bound phosphopyridoxyllysine indicated one mole of pyridoxal-P per mole of enzyme under conditions that produced maximal activation. A single [3H]phosphopyridoxyllysine containing peptide was isolated by high performance liquid chromatography after enzymatic cleavage of the modified enzyme. This 34 residue peptide exhibited considerable sequence homology to the region comprising residues 242 to 275 of human liver ALR1 and a similar region in rat lens ALR2, human muscle ALR2 and human placental ALR2. The activation of ALR2 via formation of a Schiff's base suggests a possible mechanism of activation of the enzyme in vivo by glucose.

Concerning the mechanism for transfer of D-glucuronate from myo-inositol oxygenase to D-glucuronate reductase

The D-glucuronate product of myo-inositol oxygenase (EC 1.13.99.1) is efficiently reduced by NADPH in the presence of either purified D-glucuronate reductase (EC 1.1.1.19), or reductase that is part of a protein aggregate that also contains the oxygenase. This occurs despite the fact that the maximum concentration of D-glucuronate that could be formed by the oxygenase under the conditions used for the coupled enzyme experiments is 7 microM, and 10 microM externally supplied D-glucuronate (Km = 7.6 mM) does not support any detectable NADPH oxidation under the reaction conditions. The most likely explanation for the results is that the uncyclized aldehyde form of D-glucuronate is the product of the oxygenase reaction, and that it diffuses into solution and is captured by the reductase before it cyclizes to the more stable but less reactive hemiacetal form.

L-myo-inosose-1 as a probable intermediate in the reaction catalyzed by myo-inositol oxygenase

In previous investigations, it was necessary to have Fe(II) and cysteine present in order to assay the catalytic activity of purified hog kidney myo-inositol oxygenase. In the present study it was found that, if this purified nonheme iron enzyme is slowly frozen in solution with glutathione and stored at -20 degrees C, it is fully active in the absence of activators if catalase is present to remove adventitious H2O2. With this simpler assay system it was possible to clarify the effects of several variables on the enzymic reaction. Thus, the maximum velocity is pH-dependent with a maximum around pH 9.5, but the apparent Km for myo-inositol (air atmosphere) remains constant at 5.0 mM throughout a broad pH range. The enzyme is quite specific for its substrate myo-inositol, is very sensitive to oxidants and reductants, but is not affected by a variety of complexing agents, nucleotides, sulfhydryl reagents, etc. In other experiments it was found that L-myo-inosose-1, a potential intermediate in the enzymic reaction, is a potent competitive inhibitor (Ki = 62 microM), while other inososes and a solution thought to contain D-glucodialdehyde, another potential intermediate, are weak inhibitors. Also, both a kinetic deuterium isotope effect (kH/kD = 2.1) and a tritium isotope effect (kH/kT = 7.5) are observed for the enzymic reaction when [1-2H]- and [1-3H]-myo-inositol are used as reactants. These latter results are considered strong evidence that the oxygenase reaction proceeds by a pathway involving L-myo-inosose-1 as an intermediate rather than by an alternative pathway that would have D-glucodialdehyde as the intermediate.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldose and aldehyde reductase exhibit isocorticosteroid reductase activity

In this paper we describe the reduction of corticosteroid metabolites containing the 17 beta-aldol side chain (isocorticosteroids) by aldose and aldehyde reductase from human tissues. Aldose reductase catalyzed the reduction of the aldehydes derived from cortisol and corticosterone at about the same rate, whereas aldehyde reductase preferentially acted on the aldehydes derived from 17-deoxycorticosteroids. At comparable rates of reduction the Michaelis constants for the best steroid aldehydes were one order of magnitude lower than for the hitherto best substrates. We propose that aldose and aldehyde reductase participate in the conversion of the corticosteroid ketol side chain to the glycol side chain via an aldol intermediate by the 'long loop' pathway proposed by Monder and Bradlow [(1977) J. Steroid Biochem. 8, 897-908].

The kinetics and mechanism of pig-liver aldehyde reductase. Comparative studies with pyridine-3-aldehyde and p-carboxybenzaldehyde

Initial-rate measurements were made of the reduction of pyridine-3-aldehyde and p-carboxybenzaldehyde by NADPH catalyzed by pig liver aldehyde reductase I. The initial velocity analysis and product inhibition data suggest that aldehyde reductase I obeys a compulsory-order mechanism with pyridine-3-aldehyde as substrate but follows a partially random-order pathway with p-carboxybenzaldehyde. The partially random-order pathway would be operative only at high concentrations of p-carboxybenzaldehyde. In both cases, aldehydes and the corresponding alcohol substrates inhibit the enzyme at high concentration. Abortive ternary complexes are shown to be formed with pyridine-3-aldehyde and with p-carboxybenzaldehyde. Dissociation of the coenzyme from the abortive ternary complex seems only to be observed with p-carboxybenzaldehyde. This study suggests overall that an enzyme kinetic mechanism may be different, depending on whether specific interactions can occur between certain amino acid residue(s) of the protein active site and substrates. Finally, the mechanism of the inhibition of pyridine-3-aldehyde reduction by diacid derivatives is discussed.

Properties of an aldose reductase from pig lens. Comparative studies of an aldehyde reductase from pig lens

Two monomeric NADPH enzymes from pig lens, an aldehyde reductase and an aldose reductase, have been characterized. The aldose reductase is obtained in a pure form. The aldehyde reductase, usually called hexonate dehydrogenase, is the same protein as that was recently isolated from pig liver [Branlant, G. and Biellmann, J.F. (1980) Eur. J. Biochem. 105, 611-621]. The aldose reductase is shown to have a number of properties in common with the aldehyde reductase, namely its physico-chemical properties, its tendency to be inhibited by quercitine derivatives and its substrate specificity. These two enzymes differ in their immunological properties. Only aldose reductase has a reactive Cys residue, localized in or near the substrate binding site. In contrast to that shown for aldehyde reductase [Branlant, G. et al. (1981) Eur. J. Biochem. 116, 505-512; Branlant, G. (1982) Eur. J. Biochem. 121, 407-411], no anion-recognition sites are in the substrate binding site of aldose reductase. The fact that also sugars are substrates for aldose reductase support the idea that this enzyme is implicated in the formation of sugar cataract as suggested by Kinoshita, J.H. et al. [J. Am. Med. Ass. 246, 257-261 (1981)]. Pig lens aldose reductase does not show homotropic cooperative effects with respect to either substrate or coenzyme.