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

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.

A review of the role of inositols in conditions of insulin dysregulation and in uncomplicated and pathological pregnancy

Inositols, a group of 6-carbon polyols, are highly bioactive molecules derived from diet and endogenous synthesis. Inositols and their derivatives are involved in glucose and lipid metabolism and participate in insulin-signaling, with perturbations in inositol processing being associated with conditions involving insulin resistance, dysglycemia and dyslipidemia such as polycystic ovary syndrome and diabetes. Pregnancy is similarly characterized by substantial and complex changes in glycemic and lipidomic regulation as part of maternal adaptation and is also associated with physiological alterations in inositol processing. Disruptions in maternal adaptation are postulated to have a critical pathophysiological role in pregnancy complications such as gestational diabetes and pre-eclampsia. Inositol supplementation has shown promise as an intervention for the alleviation of symptoms in conditions of insulin resistance and for gestational diabetes prevention. However, the mechanisms behind these affects are not fully understood. In this review, we explore the role of inositols in conditions of insulin dysregulation and in pregnancy, and identify priority areas for research. We particularly examine the role and function of inositols within the maternal-placental-fetal axis in both uncomplicated and pathological pregnancies. We also discuss how inositols may mediate maternal-placental-fetal cross-talk, and regulate fetal growth and development, and suggest that inositols play a vital role in promoting healthy pregnancy.

Circulating Metabolites and Lipids Are Associated to Diabetic Retinopathy in Individuals With Type 1 Diabetes

Omics-based methods may provide new markers associated to diabetic retinopathy (DR). We investigated a wide omics panel of metabolites and lipids related to DR in type 1 diabetes. Metabolomic analyses were performed using two-dimensional gas chromatography with time-of-flight mass spectrometry and lipidomic analyses using an ultra-high-performance liquid chromatography quadruple time-of-flight mass spectrometry method in 648 individuals with type 1 diabetes. Subjects were subdivided into no DR, mild nonproliferative DR (NPDR), moderate NPDR, proliferative DR, and proliferative DR with fibrosis. End points were any progression of DR, onset of DR, and progression from mild to severe DR tracked from standard ambulatory care and investigated using Cox models. The cohort consisted of 648 participants aged a mean of 54.4 ± 12.8 years, 55.5% were men, and follow-up was 5.1-5.5 years. Cross-sectionally, 2,4-dihydroxybutyric acid (DHBA), 3,4-DHBA, ribonic acid, ribitol, and the triglycerides 50:1 and 50:2 significantly correlated (P < 0.042) to DR stage. Longitudinally, higher 3,4-DHBA was a risk marker for progression of DR (n = 133) after adjustment (P = 0.033). We demonstrated multiple metabolites being positively correlated to a higher grade of DR in type 1 diabetes and several triglycerides being negatively correlated. Furthermore, higher 3,4-DHBA was an independent risk marker for progression of DR; however, confirmation is required.

Sequence-based bioprospecting of myo-inositol oxygenase (Miox) reveals new homologues that increase glucaric acid production in Saccharomyces cerevisiae

myo-Inositol oxygenase (Miox) is a rate-limiting enzyme for glucaric acid production via microbial fermentation. The enzyme converts myo-inositol to glucuronate, which is further converted to glucaric acid, a natural compound with industrial uses that range from detergents to pharmaceutical synthesis to polymeric materials. More than 2,000 Miox sequences are available in the Uniprot database but only thirteen are classified as reviewed in Swiss-Prot (August 2019). In this study, sequence similarity networks were used to identify new homologues to be expressed in Saccharomyces cerevisiae for glucaric acid production. The expression of four homologues did not lead to product formation. Some of these enzymes may have a defective "dynamic lid" - a structural feature important to close the reaction site - which might explain the lack of activity. Thirty-one selected Miox sequences did allow for product formation, of which twenty-five were characterized for the first time. Expression of Talaromyces marneffei Miox led to the accumulation of 1.76 ± 0.33 g glucaric acid/L from 20 g glucose/L and 10 g/L myo-inositol. Specific glucaric acid titer with TmMiox increased 44 % compared to the often-used Arabidopsis thaliana variant AtMiox4 (0.258 vs. 0.179 g glucaric acid/g biomass). AtMiox4 activity decreased from 12.47 to 0.40 nmol/min/mg protein when cells exited exponential phase during growth on glucose, highlighting the importance of future research on Miox stability in order to further improve microbial production of glucaric acid.

The Kidney Specific Protein myo-Inositol Oxygenase, a Potential Biomarker for Diabetic Nephropathy

BACKGROUND/AIMS

Renal tubular injury plays an important role in the progression of diabetic nephropathy (DN). However, there is a lack of specific biomarkers for tubular damage in incipient DN. We have evaluated the role of myo-inositol oxygenase (MIOX) in the tubular injury of DN, but whether it could serve as a new biomarker for the early diagnosis of DN is unclear.

METHODS

Ninety patients with type 2 diabetes mellitus (T2DM) were divided into normoalbuminuria, microalbuminuria and macroalbuminuria groups. Fifteen patients from the last group were pathologically diagnosed as type 2 DN (T2DN), and fifteen patients with minimal change disease served as a control group. The expression of MIOX and silent information regulator 1 (Sirt1) in renal biopsies was determined by immunohistochemistry (IHC), and serum/urine MIOX, Sirt1, KIM-1 and NGAL were measured using enzyme-linked immunosorbent assays (ELISAs). Spearman's correlation and multiple regression analyses were carried out for statistical analyses.

RESULTS

Compared with the controls, MIOX expression was significantly increased in the renal tissues of T2DN patients, and was positively correlated with tubulointerstitial lesions and renal ROS production but inversely correlated with Sirt1 expression. In addition, the serum and urine MIOX were significantly increased and gradually elevated with the increasing of UACR. Interestingly, elevated MIOX levels in serum and urine were found in diabetic patients without early signs of glomerular damage (normoalbuminuric group). Further multivariate regression analysis showed that sMIOX and uMIOX correlated significantly with HbA1c, serum creatinine and logUACR, respectively.

CONCLUSION

These data indicate that increased MIOX expression in the kidney contributes to tubular damage in DN. The concentration of MIOX in the serum and urine may serve as a new biomarker for the early diagnosis of DN.

Does myo-inositol oxygenase, the only enzyme to catalyze myo-inositol in vivo, play a role in the etiology of polycystic ovarian syndrome?

In polycystic ovary syndrome (PCOS), myo-inositol (MI) supplements have shown many beneficial effects. In this study, therefore, we aimed to investigate the serum level of myo-inositol oxygenase (MIOX), which is the only enzyme catalyzing MI in vivo, in patients with PCOS. Serum MIOX enzyme levels and other laboratory parameters were compared between sixty patients, who were diagnosed with PCOS for the first time, and sixty healthy individuals at similar age and sex. MIOX serum levels were not different between two groups (p = 0.7428). MIOX median and 95% CI were 19.4 and 10.6-39.1 in the control group and 16.4 and 7.6-46.2 in the patient group respectively. Demographic data, biochemical and hematological parameters, hormone parameters were not different except from the lymphocyte count between the two groups. Lymphocyte count was higher in the patient group. Although the ratio of LH/FSH was higher in the patient group, it was not statistically significant. Our results suggest that serum MIOX levels do not change in PCOS. It was, therefore, concluded that MI deficiency observed in PCOS was not related to the level of MIOX enzyme which cleaves MI.

Evaluation of the effect of Retrograde Intrarenal Surgery with Myo-Inositol Oxygenase

Objective:

To investigate the effect of retrograde intra-renal surgery (RIRS) on kidneys using the myo-inositol oxygenase (MIOX) enzyme. MIOX is a renal tubular-specific novel marker for the early diagnosis of acute kidney injury.

Methods:

A total of twenty seven individuals that had undergone RIRS to treat kidney stones were included in the study. Biochemical tests were performed on serum samples collected immediately before RIRS (hour 0) and at the 6th and 24th hours after the surgery.

Results:

The creatinine value at hour 6 was lower than the baseline (hour 0) value (p = 0.0305). Cystatin C at hour 6 was lower than the value measured at hour 24 (p = 0.0142). Similarly, MIOX was lower at hour 6 compared to hour 24 (p = 0.0214). MIOX/creatinine at hour 6 was lower than the value calculated at hour 24 (p = 0.0348). The basal values of MIOX and creatinine were found to have a positive correlation (correlation coefficient r = 0.5946, p = 0.0035).

Conclusions:

Similar to the serum creatinine, the serum MIOX level provides information about kidney functions. RIRS was confirmed to be a safe procedure for the treatment of acute kidney injury with no negative effects on the kidneys.

Changes in Immunoreactivity of Sensory Substances within the Enteric Nervous System of the Porcine Stomach during Experimentally Induced Diabetes

One of the most frequently reported disorders associated with diabetes is gastrointestinal (GI) disturbance. Although pathogenesis of these complications is multifactorial, the complicity of the enteric nervous system (ENS) in this respect has significant importance. Therefore, this paper analysed changes in substance P- (SP-), calcitonin gene-related peptide- (CGRP-), and leu5-enkephalin- (L-ENK-) like immunoreactivity (LI) in enteric stomach neurons caused by chemically induced diabetes in a porcine model. Using double immunofluorescent labelling, it was found that acute hyperglycaemia led to significant changes in the chemical coding of stomach enteric neurons. Generally, the response to artificially inducted diabetes depended on the "kind" of enteric plexus as well as the stomach region studied. A clear increase in the percentage of neurons immunoreactive to SP and CGRP was visible in the myenteric plexus (MP) in the antrum, corpus, and pylorus as well as in the submucosal plexus (SmP) in the corpus. For L-ENK, an increase in the number of L-ENK-LI neurons was observed in the MP of the antrum and SmP in the corpus, while in the MP of the corpus and pylorus, a decrease in the percentage of L-ENK-LI neurons was noted.

Contribution of myo-inositol oxygenase in AGE:RAGE-mediated renal tubulointerstitial injury in the context of diabetic nephropathy

Advanced glycation end products (AGEs) play a role in pathogenesis of diabetic nephropathy (DN). Myo-inositol oxygenase (MIOX) has been implicated in tubulointerstitial injury in the context of DN. We investigated the effect of AGEs on MIOX expression and delineated mechanisms that lead to tubulointerstitial injury. The status of MIOX, RAGE, and relevant cellular signaling pathways activated following AGE:RAGE interaction was examined in tubular cells and kidneys of AGE-BSA-treated mice. A solid-phase assay revealed an enhanced binding of RAGE with AGE-BSA, AGE-laminin, and AGE-collagen IV. The cells treated with AGE-BSA had increased MIOX activity/expression and promoter activity. This was associated with activation of various signaling kinases of phosphatidylinositol 3-kinase (PI3K)-AKT pathway and increased expression of NF-κB, transforming growth factor (TGF)-β, and fibronectin, which was negated with the treatment of MIOX/RAGE- small interfering (si) RNA. Concomitant with MIOX upregulation, there was an increased generation of reactive oxygen species (ROS), which could be abrogated with MIOX/RAGE- siRNA treatment. The kidneys of mice treated with AGE-BSA had significantly high urinary A/C ratio, upregulation of MIOX, RAGE and NF-κB, along with influx of monocytes into the tubulointerstitium, increased the expression of MCP-1, IL-6, and fibronectin and increased the generation of ROS. Such perturbations were abrogated with the concomitant treatment of inhibitors MIOX or RAGE (d-glucarate and FPS-ZM1). These studies support a role of AGE:RAGE interaction in the activation of PI3K-AKT pathway and upregulation of MIOX, with excessive generation of ROS, increased expression of NF-κB, inflammatory cytokines, TGF-β, and fibronectin. Collectively, these observations highlight the relevance of the biology of MIOX in the contribution toward tubulointerstitial injury in DN.

Molecular cloning of the myo-inositol oxygenase gene from the kidney of baboons

The enzyme myo-Inositol oxygenase (MIOX) is also termed ALDRL6. It is a kidney-specific member of the aldo-keto reductase family. MIOX catalyzes the first reaction involved in the myo-inositol metabolism signaling pathway and is fully expressed in mammalian tissues. MIOX catalyzes the oxidative cleavage of myo-Inositol and its epimer, D-chiro-Inositol to D-glucuronate. The dioxygen-dependent cleavage of the C6 and C1 bond in myo-Inositol is achieved by utilizing the Fe₂₊/Fe₃₊ binuclear iron center of MIOX. This enzyme has also been implicated in the complications of diabetes, including diabetic nephropathy. The MIOX gene was amplified with reverse transcription-polymerase chain reaction from baboon tissue samples, and the product was cloned and sequenced. MIOX expression in the baboon kidney is described in the present study. The percentages of nucleotide and amino acid similarities between baboons and humans were 95 and 96%, respectively. The MIOX protein of the baboon may be structurally identical to that of humans. Furthermore, the evolutionary changes, which have affected these sequences, have resulted from purifying forces.

Proteomic Assessment of Tanshinone II A Sodium Sulfonate on Doxorubicin Induced Nephropathy

Although doxorubicin (DXR) is an important antineoplastic agent, the serious toxicity mediated by the production of reactive oxygen species has remained a considerable clinical problem. Our hypothesis is that tanshinone II A sodium sulfonate (TSNIIA-SS), which holds significant effects against oxidative stress, protects against DXR-induced nephropathy. Firstly, the antioxidative effects of TSNIIA-SS were confirmed using oxygen radicals absorbance capacities (ORAC) assay in vitro. Then, DXR nephropathy was induced by repeated DXR treatment and verified by kidney index (20.76 ± 3.04 mg/mm versus 14.76 ± 3.04 mg/mm, p < 0.001) and histochemical stain. The mice were randomized into three groups: Control group, DXR group and DXR-TSNIIA-SS group. TSNIIA-SS treatment not only improved DXR lesion identified by histochemical stain, but also regulated the expression of several proteins related with the cytoskeleton, oxidative stress and protein synthesis or degradation detected by two-dimensional electrophoresis (2-DE). These data have provided the evidence that TSNIIA-SS is a protective agent against DXR-induced nephropathy.

Polymorphisms of myo-inositol oxygenase gene are associated with Type 1 diabetes mellitus

Myo-inositol oxygenase (MIOX) is the first and rate-limiting enzyme in myo-inositol (MI) metabolism pathway. The increase in MIOX enzyme activity is in proportion to serum glucose concentrations and may be responsible for the MI depletion found in the diabetic complications. The aim was to investigate whether single nucleotide polymorphisms (SNPs) in the MIOX gene are associated with Type 1 diabetes mellitus (T1D) and its complications. Four hundred thirty Caucasian patients with T1D were recruited: 172 patients had diabetic nephropathy, 140 had diabetic retinopathy/neuropathy, 118 patients had diabetes for ≥20 years without microvascular complications and 224 were normal controls. Three SNPs, rs761745 (C/T), and rs2232873 (A/G) in the promoter and rs1055271 (C/G) in the 3'-untranslated were genotyped commercially. The frequencies of the CC genotype (0.36 vs. 0.44; P=.034) and C allele (0.60 vs. 0.68; P=.011) of rs761745 were significantly lower in patients with T1D compared with normal controls. Patients with T1D had a decreased frequency of the combination genotypes of CC (rs761745), GG (rs2232873) and GC (rs1055271) compared with the normal controls (0.13 vs. 0.22, P=.0027, Pc=0.014). The haplotypes with C/G/G and C/G/C were less common in patients with T1D compared to normal controls (0.59 vs. 0.70, P=.021) and the haplotypes with T/G/C and T/G/G ware more common in patients with T1D compared to normal controls (0.37 vs. 0.26; P=.021). In summary, our results demonstrated that the polymorphism (rs761745) in the promoter region of MIOX gene may be associated with the development of T1D in our studied population.

The role of inositol and the principles of labelling, extraction, and analysis of inositides in mammalian cells

Inositides have an important impact on diverse areas of cellular regulation. However, since this area has grown exponentially from the mid 1980s onwards, many workers find themselves relatively new to the field. In this chapter, we establish a broad foundation for the rest of the book by covering some important principles of inositide methodologies. The focus is especially directed to those methods or aspects of methodology not covered in detail in other chapters. This includes the often neglected influence of the inositide precursor, inositol, and important background information relating to the labelling and extraction of inositides from cells and tissues. This introductory section also gives a "birds eye" view of important methods and protocols found within this volume and hopefully acts as a touchstone to assess which of the methodologies described within this book is most appropriate for your particular study(ies) of inositides.

Morphological Changes and Immunohistochemical Expression of RAGE and its Ligands in the Sciatic Nerve of Hyperglycemic Pig (Sus Scrofa)

The aim of our project was to study the effect of streptozotocin (STZ)-induced hyperglycemia on sciatic nerve morphology, blood plasma markers and immunohistochemical expression of RAGE (the Receptor for Advanced Glycation End-products), and its ligands-S100B and Carboxymethyl Lysine (CML)-advanced glycation endproduct (AGE) in the laboratory pig. Six months after STZ-injections, blood plasma measurements, morphometric analysis of sciatic nerve fiber density, immunofluorescent distribution of potential molecular neuropathy contributors, ELISA measurement of plasma AGE level and HPLC analysis of sciatic nerve levels of one of the pre-AGE and the glycolysis intermediate products-methyl-glyoxal (MG) were performed. The results of our study revealed that STZ-injected animals displayed elevated levels of plasma glucose, gamma glutamyl transferase (GGT) and triglycerides. The sciatic nerve of STZ-injected pigs revealed significantly lower numbers of small-diameter myelinated fibers, higher immunoreactivity for RAGE and S100B and increased levels of MG as compared to control animals. Our results correspond to clinical findings in human patients with hyperglycemia/diabetes-evoked peripheral neuropathy and suggest that the domestic pig may be a suitable large animal model for the study of mechanisms underlying hyperglycemia-induced neurological complications in the peripheral nerve and may serve as a relevant model for the pre-clinical assessment of candidate drugs in neuropathy.

The tissue and plasma concentration of polyols and sugars in sheep intrauterine growth retardation

In an ovine model of placental insufficiency-induced intrauterine growth retardation (PI-IUGR), characterized by hypoxia, hypoglycemia and a significant reduction in fetal weight, we assessed alterations in fetal and placental polyols. Arterial maternal–fetal concentration differences of glucose and mannose were greater in the PI-IUGR fetus; glucose: C ( n = 7), 2.68 ± 0.14 mmol/L versus PI-IUGR ( n = 9), 3.18 ± 0.16 mmol/L ( P < 0.02) and mannose: C, 42.9 ± 8.1 μmol/L versus PI-IUGR, 68.5 ± 19.1 μmol/L ( P < 0.001). For PI-IUGR fetuses, fetal arterial plasma myo-inositol concentrations were significantly increased ( P < 0.001). The concentrations of sorbitol, glucose and fructose were significantly reduced ( P < 0.03, 0.01, 0.02, respectively). The cotyledons of IUGR placentas had a significantly increased concentration of myo-inositol ( P < 0.003) and decreased concentrations of sorbitol, fructose and glycerol ( P < 0.01, 0.02, 0.01, respectively). Fetal hepatic concentrations of sorbitol ( P < 0.001) and fructose ( P < 0.03) were also significantly reduced. These profound changes in both placental and fetal concentrations of polyols and sugars in sheep PI-IUGR pregnancies support the conclusion that within the PI-IUGR placenta there is an increased flux through the glucose 6-P:inositol 1-P cyclase system and decreased flux through the polyol dehydrogenase system, leading to increased placental myo-inositol production and decreased sorbitol production. The decreased placental supply of sorbitol to the fetal liver may lead to decreased fetal hepatic fructose production. These observations highlight that, in association with hypoxic and hypoglycemic PI-IUGR fetuses, there are major placental and fetal alterations in polyol production. The manner in which these alterations in fetoplacental carbohydrate metabolism contribute to the pathophysiology of PI-IUGR is currently unknown.

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.

Computational modeling and in silico analysis of differential regulation of myo-inositol catabolic enzymes in Cryptococcus neoformans

Background

Inositol is a key cellular metabolite for many organisms. Cryptococcus neoformans is an opportunistic pathogen which primarily infects the central nervous system, a region of high inositol concentration, of immunocompromised individuals. Through the use of myo-inositol oxygenase C. neoformans can catabolize inositol as a sole carbon source to support growth and viability.

Results

Three myo-inositol oxygenase gene sequences were identified in the C. neoformans genome. Differential regulation was suggested by computational analyses of the three gene sequences. This included examination of the upstream regulatory regions, identifying ORE/TonE and UAS_INO sequences, conserved introns/exons, and in frame termination sequences. Homology modeling of the proteins encoded by these genes revealed key differences in the myo-inositol active site.

Conclusion

The results suggest there are two functional copies of the myo-inositol oxygenase gene in the C. neoformans genome. The functional genes are differentially expressed in response to environmental inositol concentrations. Both the upstream regulatory regions of the genes and the structure of the specific proteins suggest that MIOX1 would function when inositol concentrations are low, whereas MIOX2 would function when inositol concentrations are high.

Structural and biophysical characterization of human myo-inositol oxygenase

Altered inositol metabolism is implicated in a number of diabetic complications. The first committed step in mammalian inositol catabolism is performed by myo-inositol oxygenase (MIOX), which catalyzes a unique four-electron dioxygen-dependent ring cleavage of myo-inositol to D-glucuronate. Here, we present the crystal structure of human MIOX in complex with myo-inosose-1 bound in a terminal mode to the MIOX diiron cluster site. Furthermore, from biochemical and biophysical results from N-terminal deletion mutagenesis we show that the N terminus is important, through coordination of a set of loops covering the active site, in shielding the active site during catalysis. EPR spectroscopy of the unliganded enzyme displays a two-component spectrum that we can relate to an open and a closed active site conformation. Furthermore, based on site-directed mutagenesis in combination with biochemical and biophysical data, we propose a novel role for Lys(127) in governing access to the diiron cluster.

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.

Purification, crystallization and preliminary crystallographic analysis of mouse myo-inositol oxygenase

Myo-inositol oxygenase (MIOX) catalyzes the novel oxidative cleavage of myo-inositol (MI) and its epimer D-chiro inositol (DCI) to D-glucuronate. MIOX utilizes an Fe(II)/Fe(III) binuclear iron centre for the dioxygen-dependent cleavage of the C1-C6 bond in MI. Despite its key role in inositol metabolism, the structural basis of its unique four-electron oxidation mechanism and its substrate specificity remain unknown. In order to answer these questions and to facilitate the use of this key enzyme for the development of new therapeutic strategies for diabetes, the mouse enzyme has been cloned, expressed in Escherichia coli, purified and crystallized from 4.4 M sodium formate. The crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 44.87, b = 77.26, c = 84.84 angstroms, and diffract to 2.8 angstroms resolution.