Melanization of Candida auris Is Associated with Alteration of Extracellular pH.

Candida auris is a recently emerged global fungal pathogen, which causes life-threatening infections, often in healthcare settings. C. auris infections are worrisome because the fungus is often resistant to multiple antifungal drug classes. Furthermore, C. auris forms durable and difficult to remove biofilms. Due to the relatively recent, resilient, and resistant nature of C. auris, we investigated whether it produces the common fungal virulence factor melanin. Melanin is a black-brown pigment typically produced following enzymatic oxidation of aromatic precursors, which promotes fungal virulence through oxidative stress resistance, mammalian immune response evasion, and antifungal peptide and pharmaceutical inactivation. We found that certain strains of C. auris oxidized L-DOPA and catecholamines into melanin. Melanization occurred extracellularly in a process mediated by alkalinization of the extracellular environment, resulting in granule-like structures that adhere to the fungus' external surface. C. auris had relatively high cell surface hydrophobicity, but there was no correlation between hydrophobicity and melanization. Melanin protected the fungus from oxidative damage, but we did not observe a protective role during infection of macrophages or Galleria mellonella larvae. In summary, C. auris alkalinizes the extracellular medium, which promotes the non-enzymatic oxidation of L-DOPA to melanin that attaches to its surface, thus illustrating a novel mechanism for fungal melanization.

Candida glabrata produces a melanin-like pigment that protects against stress conditions encountered during parasitism.

Aim: Melanin has been linked to pathogenesis in several fungi. They often produce melanin-like pigments in the presence of L-dihydroxyphenylalanine (L-DOPA), but this is poorly studied in Candida glabrata. Methods & materials:C. glabrata was grown in minimal medium with or without L-DOPA supplementation and submitted to a chemical treatment with denaturant and hot acid. Results:C. glabrata turned black when grown in the presence of L-DOPA, whereas cells grown without L-DOPA supplementation remained white. Biophysical properties demonstrated that the pigment was melanin. Melanized C. glabrata cells were effectively protected from azoles and amphotericin B, incubation at 42°C and macrophage killing. Conclusion: In the presence of L-DOPA, C. glabrata produces melanin, increases antifungal resistance and enhances host survival.

Aim: Melanin is a pigment that can help fungi to cause disease. Fungi often produce melanin-like pigments in the presence of L-dihydroxyphenylalanine (L-DOPA), but this is poorly studied in Candida glabrata, a yeast species that can cause human disease. Methods & materials:C. glabrata was grown in minimal medium with or without L-DOPA supplementation and submitted to a chemical treatment to isolate melanin. Results:C. glabrata turned black when grown in the presence of L-DOPA, whereas cells grown without L-DOPA supplementation remained white. Several experiments demonstrated that the black pigment was melanin. Melanized C. glabrata cells were effectively protected from antifungal drugs, incubation at 42°C and killing by cells of the immune system. Conclusion: In the presence of L-DOPA, C. glabrata produces melanin, increases antifungal resistance and has enhanced survival in contact with immunologic defense cells.

Narrow-Spectrum Antibiotic Targeting of the Radical SAM Enzyme MqnE in Menaquinone Biosynthesis.

Antibiotic resistance continues to spread at an alarming rate, outpacing the introduction of new therapeutics and threatening to globally undermine health care. There is a crucial need for new strategies that selectively target specific pathogens while leaving the majority of the microbiome untouched, thus averting the debilitating and sometimes fatal occurrences of opportunistic infections. To address these challenges, we have adopted a unique strategy that focuses on oxygen-sensitive proteins, an untapped set of therapeutic targets. MqnE is a member of the radical S-adenosyl-l-methionine (RS) superfamily, all of which rely on an oxygen-sensitive [4Fe-4S] cluster for catalytic activity. MqnE catalyzes the conversion of didehydrochorismate to aminofutalosine in the essential menaquinone biosynthetic pathway present in a limited set of species, including the gut pathogen Helicobacter pylori (Hp), making it an attractive target for narrow-spectrum antibiotic development. Indeed, we show that MqnE is inhibited by the mechanism-derived 2-fluoro analogue of didehydrochorismate (2F-DHC) due to accumulation of a radical intermediate under turnover conditions. Structures of MqnE in the apo and product-bound states afford insight into its catalytic mechanism, and electron paramagnetic resonance approaches provide direct spectroscopic evidence consistent with the predicted structure of the radical intermediate. In addition, we demonstrate the essentiality of the menaquinone biosynthetic pathway and unambiguously validate 2F-DHC as a selective inhibitor of Hp growth that exclusively targets MqnE. These data provide the foundation for designing effective Hp therapies and demonstrate proof of principle that radical SAM proteins can be effectively leveraged as therapeutic targets.

Structure-function analyses of the G729R 2-oxoadipate dehydrogenase genetic variant associated with a disorder of l-lysine metabolism.

2-Oxoadipate dehydrogenase (E1a, also known as DHTKD1, dehydrogenase E1, and transketolase domain-containing protein 1) is a thiamin diphosphate-dependent enzyme and part of the 2-oxoadipate dehydrogenase complex (OADHc) in l-lysine catabolism. Genetic findings have linked mutations in the DHTKD1 gene to several metabolic disorders. These include α-aminoadipic and α-ketoadipic aciduria (AMOXAD), a rare disorder of l-lysine, l-hydroxylysine, and l-tryptophan catabolism, associated with clinical presentations such as developmental delay, mild-to-severe intellectual disability, ataxia, epilepsy, and behavioral disorders that cannot currently be managed by available treatments. A heterozygous missense mutation, c.2185G→A (p.G729R), in DHTKD1 has been identified in most AMOXAD cases. Here, we report that the G729R E1a variant when assembled into OADHc in vitro displays a 50-fold decrease in catalytic efficiency for NADH production and a significantly reduced rate of glutaryl-CoA production by dihydrolipoamide succinyl-transferase (E2o). However, the G729R E1a substitution did not affect any of the three side-reactions associated solely with G729R E1a, prompting us to determine the structure-function effects of this mutation. A multipronged systematic analysis of the reaction rates in the OADHc pathway, supplemented with results from chemical cross-linking and hydrogen-deuterium exchange MS, revealed that the c.2185G→A DHTKD1 mutation affects E1a-E2o assembly, leading to impaired channeling of OADHc intermediates. Cross-linking between the C-terminal region of both E1a and G729R E1a with the E2o lipoyl and core domains suggested that correct positioning of the C-terminal E1a region is essential for the intermediate channeling. These findings may inform the development of interventions to counter the effects of pathogenic DHTKD1 mutations.

The active site of the Mycobacterium tuberculosis branched-chain amino acid biosynthesis enzyme dihydroxyacid dehydratase contains a 2Fe-2S cluster.

Iron-sulfur clusters are protein cofactors with an ancient evolutionary origin. These clusters are best known for their roles in redox proteins such as ferredoxins, but some iron-sulfur clusters have nonredox roles in the active sites of enzymes. Such clusters are often prone to oxidative degradation, making the enzymes difficult to characterize. Here we report a structural and functional characterization of dihydroxyacid dehydratase (DHAD) from Mycobacterium tuberculosis (Mtb), an essential enzyme in the biosynthesis of branched-chain amino acids. Conducting this analysis under fully anaerobic conditions, we solved the DHAD crystal structure, at 1.88 Å resolution, revealing a 2Fe-2S cluster in which one iron ligand is a potentially exchangeable water molecule or hydroxide. UV and EPR spectroscopy both suggested that the substrate binds directly to the cluster or very close to it. Kinetic analysis implicated two ionizable groups in the catalytic mechanism, which we postulate to be Ser-491 and the iron-bound water/hydroxide. Site-directed mutagenesis showed that Ser-491 is essential for activity, and substrate docking indicated that this residue is perfectly placed for proton abstraction. We found that a bound Mg2+ ion 6.5 Å from the 2Fe-2S cluster plays a key role in substrate binding. We also identified a putative entry channel that enables access to the cluster and show that Mtb-DHAD is inhibited by a recently discovered herbicide, aspterric acid, that, given the essentiality of DHAD for Mtb survival, is a potential lead compound for the design of novel anti-TB drugs.

Human 2-Oxoglutarate Dehydrogenase and 2-Oxoadipate Dehydrogenase Both Generate Superoxide/H2O2 in a Side Reaction and Each Could Contribute to Oxidative Stress in Mitochondria.

According to recent findings, the human 2-oxoglutarate dehydrogenase complex (hOGDHc) could be an important source of the reactive oxygen species in the mitochondria and could contribute to mitochondrial abnormalities associated with multiple neurodegenerative diseases, including Alzheimer's disease, Huntington disease, and Parkinson's disease. The human 2-oxoadipate dehydrogenase (hE1a) is a novel protein, which is encoded by the DHTKD1 gene. Both missence and nonsense mutations were identified in the DHTKD1 that lead to alpha-aminoadipic and alpha-oxoadipic aciduria, a metabolic disorder with a wide variety of the neurological abnormalities, and Charcot-Marie-Tooth disease type 2Q, an inherited neurological disorder affecting the peripheral nervous system. Recently, the rare pathogenic mutations in DHTKD1 and an increased H2O2 production were linked to the genetic ethiology of Eosinophilic Esophagitis (EoE), a chronic allergic inflammatory esophageal disorder. In view of the importance of hOGDHc in the tricarboxylic acid cycle (TCA cycle) and hE1a on the L-lysine, L-hydroxylysine and L-tryptophan degradation pathway in mitochondria, and to enhance our current understanding of the mechanism of superoxide/H2O2 generation by hOGDHc, and by human 2-oxoadipate dehydrogenase complex (hOADHc), this review focuses on several novel and unanticipated recent findings in vitro that emerged from the Jordan group's research. Most significantly, the hE1o and hE1a now join the hE3 as being able to generate the superoxide/H2O2 in mitochondria.

Semisynthetic supra plasma expanders: a new class of therapeutics to improve microcircualtion in sickle cell anaemia.

Compromised microcirculation and endothelial dysfunction are hallmarks of sickle cell disease (SCD). EAF PEG Haemoglobin (Hb) and EAF PEG Albumin (Alb) represent a novel class of semisynthetic colloidal supra plasma expanders that improve microcirculation. The therapeutic activity of supra plasma expanders to attenuate vaso-occlusion and restore the haemodynamic functions in patients with SCD has been investigated using NY1DD, a transgenic mouse model of mild SCD without anaemia. Vaso-occlusion and perturbation of haemodynamics are amplified in NY1DD by hypoxia-reoxygenation protocol. EAF P5K6 Alb and Alb T12 (Alb conjugated with 12 copies of antioxidant tempo) attenuate vaso-occlusion when infused at the start of reoxygenation. However, only EAF PEG Alb restores haemodynamics close to levels in control C57BL. EAF P5K6 Alb-T12, active plasma expander conjugated with antioxidant, completely clears vaso-occlusion and restores normal haemodynamics. EAF PEG Hb also completely clears vaso-occlusion and restores normal haemodynamics. Pretreating NY1DD with EAF PEG Hb protects it from hypoxia reoxygenation-induced damages. EAF P5K6 Alb T12 attenuates the endothelial dysfunction in S + S Antilles mice as reflected by the vasodilatory response of its arteries and arterioles to vaso-dilators. Active plasma expanders are novel therapeutics to restore normal haemodynamics in SCD patients to improve tissue oxygenation during episodes of painful vaso-occlusive crisis. Abbreviations: 2-IT: 2-immothiolane; Mal-T: 4-Maleimido tempo; Alb: human serum albumin (HSA); Alb-T12: human albumin conjugated with 12 copies of tempo; EAF: extension arm facilitated; EAF PEG Hb: extension arm facilitated PEGylated haemoglobin; EAF PEG Alb: extension arm facilitated PEGylated albumin; EAF P3K6 Hb: extension arm facilitated PEGylated haemoglobin conjugated with 6 copies of PEG3K; EAF P5K6 Alb T12: extension arm facilitated PEGylated albumin conjugated with 6 copies of PEG5K and 12 copies of tempo; Hb: haemoglobin; HAS: human serum albumin (Alb); PEG: polyethylene glycol; MP4: MalPEG Hb, is formulated at 4.2 g/dL in lactated Ringer's solution, a product of Sangart; SCD: sickle cell disease; NO: nitric oxide; SEC: size exclusive chromatography; Vrbc: red cell velocity; Q: volumetric flow rates, Q; SNP: sodium nitroprusside.

Oxidative decarboxylation of pyruvate by 1-deoxy-d-xyulose 5-phosphate synthase, a central metabolic enzyme in bacteria.

The underexploited antibacterial target 1-deoxy-d-xyluose 5-phosphate (DXP) synthase catalyzes the thiamin diphosphate (ThDP)-dependent formation of DXP from pyruvate and d-glyceraldehyde 3-phosphate (d-GAP). DXP is an essential intermediate in the biosynthesis of ThDP, pyridoxal phosphate, and isoprenoids in many pathogenic bacteria. DXP synthase catalyzes a distinct mechanism in ThDP decarboxylative enzymology in which the first enzyme-bound pre-decarboxylation intermediate, C2α-lactyl-ThDP (LThDP), is stabilized by DXP synthase in the absence of d-GAP, and d-GAP then induces efficient LThDP decarboxylation. Despite the observed LThDP accumulation and lack of evidence for C2α-carbanion formation in the absence of d-GAP, CO2 is released at appreciable levels under these conditions. Here, seeking to resolve these conflicting observations, we show that DXP synthase catalyzes the oxidative decarboxylation of pyruvate under conditions in which LThDP accumulates. O2-dependent LThDP decarboxylation led to one-electron transfer from the C2α-carbanion/enamine to O2, with intermediate ThDP-enamine radical formation, followed by peracetic acid formation en route to acetate. Thus, LThDP formation and decarboxylation and DXP formation were studied under anaerobic conditions. Our results support a model in which O2-dependent LThDP decarboxylation and peracetic acid formation occur in the absence of d-GAP, decreasing the levels of pyruvate and O2 in solution. The relative pyruvate and O2 concentrations then dictate the extent of LThDP accumulation, and its buildup can be observed when [pyruvate] > [O2]. The finding that O2 acts as a structurally distinct trigger of LThDP decarboxylation supports the hypothesis that a mechanism involving small molecule-dependent LThDP decarboxylation equips DXP synthase for diverse, yet uncharacterized cellular functions.

Evidence for functional and regulatory cross-talk between the tricarboxylic acid cycle 2-oxoglutarate dehydrogenase complex and 2-oxoadipate dehydrogenase on the l-lysine, l-hydroxylysine and l-tryptophan degradation pathways from studies in vitro.

Herein are reported findings in vitro suggesting both functional and regulatory cross-talk between the human 2-oxoglutarate dehydrogenase complex (hOGDHc), a key regulatory enzyme within the tricarboxylic acid cycle (TCA cycle), and a novel 2-oxoadipate dehydrogenase complex (hOADHc) from the final degradation pathway of l-lysine, l-hydroxylysine and l-tryptophan. The following could be concluded from our studies by using hOGDHc and hOADHc assembled from their individually expressed components in vitro: (i) Different substrate preferences (kcat/Km) were displayed by the two complexes even though they share the same dihydrolipoyl succinyltransferase (hE2o) and dihydrolipoyl dehydrogenase (hE3) components; (ii) Different binding modes were in evidence for the binary hE1o-hE2o and hE1a-hE2o subcomplexes according to fluorescence titrations using site-specifically labeled hE2o-derived proteins; (iii) Similarly to hE1o, the hE1a also forms the ThDP-enamine radical from 2-oxoadipate (electron paramagnetic resonance detection) in the oxidative half reaction; (iv) Both complexes produced superoxide/H2O2 from O2 in the reductive half reaction suggesting that hE1o, and hE1a (within their complexes) could both be sources of reactive oxygen species generation in mitochondria from 2-oxoglutarate and 2-oxoadipate, respectively; (v) Based on our findings, we speculate that hE2o can serve as a trans-glutarylase, in addition to being a trans-succinylase, a role suggested by others; (vi) The glutaryl-CoA produced by hOADHc inhibits hE1o, as does succinyl-CoA, suggesting a regulatory cross-talk between the two complexes on the different metabolic pathways.

The mitochondrial 2-oxoadipate and 2-oxoglutarate dehydrogenase complexes share their E2 and E3 components for their function and both generate reactive oxygen species.

Herein are reported unique properties of the novel human thiamin diphosphate (ThDP)-dependent enzyme 2-oxoadipate dehydrogenase (hE1a), known as dehydrogenase E1 and transketolase domain-containing protein 1 that is encoded by the DHTKD1 gene. It is involved in the oxidative decarboxylation of 2-oxoadipate (OA) to glutaryl-CoA on the final degradative pathway of L-lysine and is critical for mitochondrial metabolism. Functionally active recombinant hE1a has been produced according to both kinetic and spectroscopic criteria in our toolbox leading to the following conclusions: (i) The hE1a has recruited the dihydrolipoyl succinyltransferase (hE2o) and the dihydrolipoyl dehydrogenase (hE3) components of the tricarboxylic acid cycle 2-oxoglutarate dehydrogenase complex (OGDHc) for its activity. (ii) 2-Oxoglutarate (OG) and 2-oxoadipate (OA) could be oxidized by hE1a, however, hE1a displays an approximately 49-fold preference in catalytic efficiency for OA over OG, indicating that hE1a is specific to the 2-oxoadipate dehydrogenase complex. (iii) The hE1a forms the ThDP-enamine radical from OA according to electron paramagnetic resonance detection in the oxidative half reaction, and could produce superoxide and H2O2 from decarboxylation of OA in the forward physiological direction, as also seen with the 2-oxoglutarate dehydrogenase hE1o component. (iv) Once assembled to complex with the same hE2o and hE3 components, the hE1o and hE1a display strikingly different regulation: both succinyl-CoA and glutaryl-CoA significantly reduced the hE1o activity, but not the activity of hE1a.

L-tyrosine induces the production of a pyomelanin-like pigment by the parasitic yeast-form of Histoplasma capsulatum.

Melanization of Histoplasma capsulatum remains poorly described, particularly in regards to the forms of melanin produced. In the present study, 30 clinical and environmental H. capsulatum strains were grown in culture media with or without L-tyrosine under conditions that produced either mycelial or yeast forms. Mycelial cultures were not melanized under the studied conditions. However, all strains cultivated under yeast conditions produced a brownish to black soluble pigment compatible with pyomelanin when grew in presence of L-tyrosine. Sulcotrione inhibited pigment production in yeast cultures, strengthening the hyphothesis that H. capsulatum yeast forms produce pyomelanin. Since pyomelanin is produced by the fungal parasitic form, this pigment may be involved in H. capsulatum virulence.

The human Krebs cycle 2-oxoglutarate dehydrogenase complex creates an additional source of superoxide/hydrogen peroxide from 2-oxoadipate as alternative substrate.

Recently, we reported that the human 2-oxoglutarate dehydrogenase (hE1o) component of the 2-oxoglutarate dehydrogenase complex (OGDHc) could produce the reactive oxygen species superoxide and hydrogen peroxide (detected by chemical means) from its substrate 2-oxoglutarate (OG), most likely concurrently with one-electron oxidation by dioxygen of the thiamin diphosphate (ThDP)-derived enamine intermediate to a C2α-centered radical (detected by Electron Paramagnetic Resonance) [Nemeria et al., 2014 [17]; Ambrus et al. 2015 [18]]. We here report that hE1o can also utilize the next higher homologue of OG, 2-oxoadipate (OA) as a substrate according to multiple criteria in our toolbox: (i) Both E1o-specific and overall complex activities (NADH production) were detected using OA as a substrate; (ii) Two post-decarboxylation intermediates were formed by hE1o from OA, the ThDP-enamine and the C2α-hydroxyalkyl-ThDP, with nearly identical rates for OG and OA; (iii) Both OG and OA could reductively acylate lipoyl domain created from dihydrolipoyl succinyltransferase (E2o); (iv) Both OG and OA gave α-ketol carboligaton products with glyoxylate, but with opposite chirality; a finding that could be of utility in chiral synthesis; (v) Dioxygen could oxidize the ThDP-derived enamine from both OG and OA, leading to ThDP-enamine radical and generation of superoxide and H2O2. While the observed oxidation-reduction with dioxygen is only a side reaction of the predominant physiological product glutaryl-CoA, the efficiency of superoxide/ H2O2 production was 7-times larger from OA than from OG, making the reaction of OGDHc with OA one of the important superoxide/ H2O2 producers among 2-oxo acid dehydrogenase complexes in mitochondria.

Paracrine Induction of HIF by Glutamate in Breast Cancer: EglN1 Senses Cysteine.

The HIF transcription factor promotes adaptation to hypoxia and stimulates the growth of certain cancers, including triple-negative breast cancer (TNBC). The HIFα subunit is usually prolyl-hydroxylated by EglN family members under normoxic conditions, causing its rapid degradation. We confirmed that TNBC cells secrete glutamate, which we found is both necessary and sufficient for the paracrine induction of HIF1α in such cells under normoxic conditions. Glutamate inhibits the xCT glutamate-cystine antiporter, leading to intracellular cysteine depletion. EglN1, the main HIFα prolyl-hydroxylase, undergoes oxidative self-inactivation in the absence of cysteine both in biochemical assays and in cells, resulting in HIF1α accumulation. Therefore, EglN1 senses both oxygen and cysteine.

Distinct Interaction Modes of the Kinesin-13 Motor Domain with the Microtubule.

Kinesins-13s are members of the kinesin superfamily of motor proteins that depolymerize microtubules (MTs) and have no motile activity. Instead of generating unidirectional movement over the MT lattice, like most other kinesins, kinesins-13s undergo one-dimensional diffusion (ODD) and induce depolymerization at the MT ends. To understand the mechanism of ODD and the origin of the distinct kinesin-13 functionality, we used ensemble and single-molecule fluorescence polarization microscopy to analyze the behavior and conformation of Drosophila melanogaster kinesin-13 KLP10A protein constructs bound to the MT lattice. We found that KLP10A interacts with the MT in two coexisting modes: one in which the motor domain binds with a specific orientation to the MT lattice and another where the motor domain is very mobile and able to undergo ODD. By comparing the orientation and dynamic behavior of mutated and deletion constructs we conclude that 1) the Kinesin-13 class specific neck domain and loop-2 help orienting the motor domain relative to the MT. 2) During ODD the KLP10A motor-domain changes orientation rapidly (rocks or tumbles). 3) The motor domain alone is capable of undergoing ODD. 4) A second tubulin binding site in the KLP10A motor domain is not critical for ODD. 5) The neck domain is not the element preventing KLP10A from binding to the MT lattice like motile kinesins.

GLP-1 Cleavage Product Reverses Persistent ROS Generation After Transient Hyperglycemia by Disrupting an ROS-Generating Feedback Loop.

The assumption underlying current diabetes treatment is that lowering the level of time-averaged glucose concentrations, measured as HbA1c, prevents microvascular complications. However, 89% of variation in risk of retinopathy, microalbuminuria, or albuminuria is due to elements of glycemia not captured by mean HbA1c values. We show that transient exposure to high glucose activates a multicomponent feedback loop that causes a stable left shift of the glucose concentration-reactive oxygen species (ROS) dose-response curve. Feedback loop disruption by the GLP-1 cleavage product GLP-1(9-36)(amide) reverses the persistent left shift, thereby normalizing persistent overproduction of ROS and its pathophysiologic consequences. These data suggest that hyperglycemic spikes high enough to activate persistent ROS production during subsequent periods of normal glycemia but too brief to affect the HbA1c value are a major determinant of the 89% of diabetes complications risk not captured by HbA1c. The phenomenon and mechanism described in this study provide a basis for the development of both new biomarkers to complement HbA1c and novel therapeutic agents, including GLP-1(9-36)(amide), for the prevention and treatment of diabetes complications.

Structural Characterization of Melanin Pigments from Commercial Preparations of the Edible Mushroom Auricularia auricula.

Many of the most widely consumed edible mushrooms are pigmented, and these have been associated with some beneficial health effects. Nevertheless, the majority of the reported compounds associated with these desirable properties are non-pigmented. We have previously reported that melanin pigment from the edible mushroom Auricularia auricula can protect mice against ionizing radiation, although no physicochemical characterization was reported. Consequently, in this study we have characterized commercial A. auricula mushroom preparations for melanin content and carried out structural characterization of isolated insoluble melanin materials using a panel of sophisticated spectroscopic and physical/imaging techniques. Our results show that approximately 10% of the dry mass of A. auricula is melanin and that the pigment has physicochemical properties consistent with those of eumelanins, including hosting a stable free radical population. Electron microscopy studies show that melanin is associated with the mushroom cell wall in a manner similar to that of melanin from the model fungus C. neoformans. Elemental analysis of melanin indicated C, H, and N ratios consistent with 5,6-dihydroxyindole-2-carboxylic acid/5,6-dihydroxyindole and 1,8-dihydroxynaphthalene eumelanin. Validation of the identity of the isolated product as melanin was achieved by EPR analysis. A. auricula melanin manifested structural differences, relative to the C. neoformans melanin, with regard to the variable proportions of alkyl chains or oxygenated carbons. Given the necessity for new oral and inexpensive radioprotective materials coupled with the commercial availability of A. auricula mushrooms, this product may represent an excellent source of edible melanin.

Evaluating the capacity to generate and preserve nitric oxide bioactivity in highly purified earthworm erythrocruorin: a giant polymeric hemoglobin with potential blood substitute properties.

The giant extracellular hemoglobin (erythrocruorin) from the earth worm (Lumbricus terrestris) has shown promise as a potential hemoglobin-based oxygen carrier (HBOC) in in vivo animal studies. An important beneficial characteristic of this hemoglobin (LtHb) is the large number of heme-based oxygen transport sites that helps overcome issues of osmotic stress when attempting to provide enough material for efficient oxygen delivery. A potentially important additional property is the capacity of the HBOC either to generate nitric oxide (NO) or to preserve NO bioactivity to compensate for decreased levels of NO in the circulation. The present study compares the NO-generating and NO bioactivity-preserving capability of LtHb with that of human adult hemoglobin (HbA) through several reactions including the nitrite reductase, reductive nitrosylation, and still controversial nitrite anhydrase reactions. An assignment of a heme-bound dinitrogen trioxide as the stable intermediate associated with the nitrite anhydrase reaction in both LtHb and HbA is supported based on functional and EPR spectroscopic studies. The role of the redox potential as a factor contributing to the NO-generating activity of these two proteins is evaluated. The results show that LtHb undergoes the same reactions as HbA and that the reduced efficacy for these reactions for LtHb relative to HbA is consistent with the much higher redox potential of LtHb. Evidence of functional heterogeneity in LtHb is explained in terms of the large difference in the redox potential of the isolated subunits.

Human 2-oxoglutarate dehydrogenase complex E1 component forms a thiamin-derived radical by aerobic oxidation of the enamine intermediate.

Herein are reported unique properties of the human 2-oxoglutarate dehydrogenase multienzyme complex (OGDHc), a rate-limiting enzyme in the Krebs (citric acid) cycle. (a) Functionally competent 2-oxoglutarate dehydrogenase (E1o-h) and dihydrolipoyl succinyltransferase components have been expressed according to kinetic and spectroscopic evidence. (b) A stable free radical, consistent with the C2-(C2α-hydroxy)-γ-carboxypropylidene thiamin diphosphate (ThDP) cation radical was detected by electron spin resonance upon reaction of the E1o-h with 2-oxoglutarate (OG) by itself or when assembled from individual components into OGDHc. (c) An unusual stability of the E1o-h-bound C2-(2α-hydroxy)-γ-carboxypropylidene thiamin diphosphate (the "ThDP-enamine"/C2α-carbanion, the first postdecarboxylation intermediate) was observed, probably stabilized by the 5-carboxyl group of OG, not reported before. (d) The reaction of OG with the E1o-h gave rise to superoxide anion and hydrogen peroxide (reactive oxygen species (ROS)). (e) The relatively stable enzyme-bound enamine is the likely substrate for oxidation by O2, leading to the superoxide anion radical (in d) and the radical (in b). (f) The specific activity assessed for ROS formation compared with the NADH (overall complex) activity, as well as the fraction of radical intermediate occupying active centers of E1o-h are consistent with each other and indicate that radical/ROS formation is an "off-pathway" side reaction comprising less than 1% of the "on-pathway" reactivity. However, the nearly ubiquitous presence of OGDHc in human tissues, including the brain, makes these findings of considerable importance in human metabolism and perhaps disease.

Catalytic intermediates of inducible nitric-oxide synthase stabilized by the W188H mutation.

Nitric-oxide synthase (NOS) catalyzes nitric oxide (NO) synthesis via a two-step process: L-arginine (L-Arg) → N-hydroxy-L-arginine → citrulline + NO. In the active site the heme is coordinated by a thiolate ligand, which accepts a H-bond from a nearby tryptophan residue, Trp-188. Mutation of Trp-188 to histidine in murine inducible NOS was shown to retard NO synthesis and allow for transient accumulation of a new intermediate with a Soret maximum at 420 nm during the L-Arg hydroxylation reaction (Tejero, J., Biswas, A., Wang, Z. Q., Page, R. C., Haque, M. M., Hemann, C., Zweier, J. L., Misra, S., and Stuehr, D. J. (2008) J. Biol. Chem. 283, 33498-33507). However, crystallographic data showed that the mutation did not perturb the overall structure of the enzyme. To understand how the proximal mutation affects the oxygen chemistry, we carried out biophysical studies of the W188H mutant. Our stopped-flow data showed that the 420-nm intermediate was not only populated during the L-Arg reaction but also during the N-hydroxy-L-arginine reaction. Spectroscopic data and structural analysis demonstrated that the 420-nm intermediate is a hydroxide-bound ferric heme species that is stabilized by an out-of-plane distortion of the heme macrocycle and a cation radical centered on the tetrahydrobiopterin cofactor. The current data add important new insights into the previously proposed catalytic mechanism of NOS (Li, D., Kabir, M., Stuehr, D. J., Rousseau, D. L., and Yeh, S. R. (2007) J. Am. Chem. Soc. 129, 6943-6951).

Compton scattering by internal shields based on melanin-containing mushrooms provides protection of gastrointestinal tract from ionizing radiation.

There is a need for radioprotectors that protect normal tissues from ionizing radiation in patients receiving high doses of radiation and during nuclear emergencies. We investigated the possibility of creating an efficient oral radioprotector based on the natural pigment melanin that would act as an internal shield and protect the tissues via Compton scattering followed by free radical scavenging. CD-1 mice were fed melanin-containing black edible mushrooms Auricularia auricila-judae before 9 Gy total body irradiation. The location of the mushrooms in the body before irradiation was determined by in vivo fluorescent imaging. Black mushrooms protected 80% of mice from the lethal dose, while control mice or those given melanin-devoid mushrooms died from gastrointestinal syndrome. The crypts of mice given black mushrooms showed less apoptosis and more cell division than those in control mice, and their white blood cell and platelet counts were restored at 45 days to preradiation levels. The role of melanin in radioprotection was proven by the fact that mice given white mushrooms supplemented with melanin survived at the same rate as mice given black mushrooms. The ability of melanin-containing mushrooms to provide remarkable protection against radiation suggests that they could be developed into oral radioprotectors.