Human erythrocytes exposure to juglone leads to an increase of superoxide anion production associated with cytochrome b5 reductase uncoupling

Cytochrome b₅ reductase is an enzyme with the ability to generate superoxide anion at the expenses of NADH consumption. Although this activity can be stimulated by cytochrome c and could participate in the bioenergetic failure accounting in apoptosis, very little is known about other molecules that may uncouple the function of the cytochrome b₅ reductase. Naphthoquinones are redox active molecules with the ability to interact with electron transfer chains. In this work, we made an inhibitor screening against recombinant human cytochrome b₅ reductase based on naphthoquinone properties. We found that 5-hydroxy-1,4-naphthoquinone (known as juglone), a natural naphthoquinone extracted from walnut trees and used historically in traditional medicine with ambiguous health and toxic outcomes, had the ability to uncouple the electron transfer from the reductase to cytochrome b₅ and ferricyanide. Upon complex formation with cytochrome b₅ reductase, juglone is able to act as an electron acceptor leading to a NADH consumption stimulation and an increase of superoxide anion production by the reductase. Our results suggest that cytochrome b₅ reductase could contribute to the measured energetic failure in the erythrocyte apoptosis induced by juglone, that is concomitant with the reactive oxygen species produced by cytochrome b₅ reductase.

Cytochrome b5 reductase is the component from neuronal synaptic plasma membrane vesicles that generates superoxide anion upon stimulation by cytochrome c

In this work, we measured the effect of cytochrome c on the NADH-dependent superoxide anion production by synaptic plasma membrane vesicles from rat brain. In these membranes, the cytochrome c stimulated NADH-dependent superoxide anion production was inhibited by antibodies against cytochrome b5 reductase linking the production to this enzyme. Measurement of the superoxide anion radical generated by purified recombinant soluble and membrane cytochrome b5 reductase corroborates the production of the radical by different enzyme isoforms. In the presence of cytochrome c, a burst of superoxide anion as well as the reduction of cytochrome c by cytochrome b5 reductase was measured. Complex formation between both proteins suggests that cytochrome b5 reductase is one of the major partners of cytochrome c upon its release from mitochondria to the cytosol during apoptosis. Superoxide anion production and cytochrome c reduction are the consequences of the stimulated NADH consumption by cytochrome b5 reductase upon complex formation with cytochrome c and suggest a major role of this enzyme as an anti-apoptotic protein during cell death.

Structural and enzymatic analysis of the cytochrome b5 reductase domain of Ulva prolifera nitrate reductase

Rapid accumulations of unattached green macroalgae, referred to as blooms, constitute ecological disasters and occur in many coastal regions. Ulva are a major cause of blooms, owing to their high nitrogen utilization capacity, which requires nitrate reductase (NR) activity; however, molecular characterization of Ulva NR remains lacking. Herein we determined the crystal structure and performed an enzymatic analysis of the cytochrome b₅ reductase domain of Ulva prolifera NR (UpCbRNR). The structural analysis revealed an N-terminal FAD-binding domain primarily consisting of six antiparallel β strands, a C-terminal NADH-binding domain forming a Rossmann fold, and a three β-stranded linker region connecting these two domains. The FAD cofactor was located in the cleft between the two domains and interacted primarily with the FAD-binding domain. UpCbRNR shares similarities in overall structure and cofactor interactions with homologs, and its catalytic ability is comparable to that of higher plant CbRNRs. Structure and sequence comparisons of homologs revealed two regions of sequence length variation potentially useful for phylogenetic analysis: one in the FAD-binding domain, specific to U. prolifera, and another in the linker region that may be used to differentiate between plant, fungi, and animal homologs. Our data will facilitate molecular-level understanding of nitrate assimilation in Ulva.

Topography of human cytochrome b5/cytochrome b5 reductase interacting domain and redox alterations upon complex formation

Cytochrome b₅ is the main electron acceptor of cytochrome b₅ reductase. The interacting domain between both human proteins has been unidentified up to date and very little is known about its redox properties modulation upon complex formation. In this article, we characterized the protein/protein interacting interface by solution NMR and molecular docking. In addition, upon complex formation, we measured an increase of cytochrome b₅ reductase flavin autofluorescence that was dependent upon the presence of cytochrome b_5. Data analysis of these results allowed us to calculate a dissociation constant value between proteins of 0.5±0.1μM and a 1:1 stoichiometry for the complex formation. In addition, a 30mV negative shift of cytochrome b₅ reductase redox potential in presence of cytochrome b₅ was also measured. These experiments suggest that the FAD group of cytochrome b₅ reductase increase its solvent exposition upon complex formation promoting an efficient electron transfer between the proteins.

Purified NADH-cytochrome b5 reductase is a novel superoxide anion source inhibited by apocynin: sensitivity to nitric oxide and peroxynitrite

Cytochrome b5 reductase (Cb5R) is a pleiotropic flavoprotein that catalyzes multiple one-electron reduction reactions with various redox partners in cells. In earlier work from our laboratory, we have shown its implication in the generation of reactive oxygen species (ROS), primarily a superoxide anion overshoot peak, which plays a major role as a triggering event for the acceleration of apoptosis in cerebellar granule neurons in culture. However, the results obtained in that work did not allow us to exclude the possibility that this superoxide anion production could be derived from Cb5R acting in concert with other cellular components. In this work, we have purified Cb5R from pig liver and we have experimentally shown that this enzyme catalyzed NADH-dependent production of superoxide anion, assayed with cytochrome c and nitroblue tetrazolium as detection reagents for this particular ROS. The basic kinetic parameters for this novel NADH-dependent activity of Cb5R at 37°C are Vmax = 3.0 ± 0.5 μmol/min/mg of purified Cb5R and KM(NADH) = 2.8 ± 0.3 μM NADH. In addition, we report that apocynin, a widely used inhibitor of nonmitochondrial ROS production in mammalian cell cultures and tissues, is a potent inhibitor of purified Cb5R activity at the concentrations used in the experiments done with cell cultures. In the presence of apocynin the KM(NADH) value of Cb5R increases, and docking simulations indicate that apocynin can bind to a site near to or partially overlapping the NADH binding site of Cb5R. Other ROS, such as nitric oxide and peroxynitrite, have inhibitory effects on purified Cb5R, providing the basis for a feedback cellular protection mechanism through modulation of excessive extramitochondrial superoxide anion production by Cb5R. Both kinetic assays and docking simulations suggest that nitric oxide-induced nitrosylation (including covalent adduction of nitroso functional groups) of Cb5R cysteines and peroxynitrite-induced tyrosine nitration and cysteine oxidation modified the conformation of the NADH binding domain leading to a decreased affinity of Cb5R for NADH.

Caveolin-rich lipid rafts of the plasma membrane of mature cerebellar granule neurons are microcompartments for calcium/reactive oxygen and nitrogen species cross-talk signaling

In previous works, we have shown that L-type voltage-operated calcium channels, N-methyl-d-aspartate receptors (NMDAr), neuronal nitric oxide synthase (nNOS) and cytochrome b5 reductase (Cb5R) co-localize within the same lipid rafts-associated nanodomains in mature cerebellar granule neurons (CGN). In this work, we show that the calcium transport systems of the plasma membrane extruding calcium from the cytosol, plasma membrane calcium pumps (PMCA) and sodium-calcium exchangers (NCX), are also associated with these nanodomains. All these proteins were found to co-immunoprecipitate with caveolin-1 after treatment with 25mM methyl-β-cyclodextrin, a lipid rafts solubilizing agent. However, the treatment of CGN with methyl-β-cyclodextrin largely attenuated the rise of cytosolic calcium induced by l-glutamate through NMDAr. Fluorescence energy transfer imaging revealed that all of them are present in sub-microdomains of a size smaller than 200nm, with a peripheral distribution of the calcium extrusion systems PMCA and NCX. Fluorescence microscopy images analysis revealed high calcium dynamic sub-microcompartments near the plasma membrane in fura-2-loaded CGN at short times after addition of l-glutamate. In addition, the close proximity between sources of nitric oxide (nNOS) and superoxide anion (Cb5R) suggests that these nanodomains are involved in the fast and efficient cross-talk between calcium and redox signaling in neurons.

Inborn defects in the antioxidant systems of human red blood cells

Red blood cells (RBCs) contain large amounts of iron and operate in highly oxygenated tissues. As a result, these cells encounter a continuous oxidative stress. Protective mechanisms against oxidation include prevention of formation of reactive oxygen species (ROS), scavenging of various forms of ROS, and repair of oxidized cellular contents. In general, a partial defect in any of these systems can harm RBCs and promote senescence, but is without chronic hemolytic complaints. In this review we summarize the often rare inborn defects that interfere with the various protective mechanisms present in RBCs. NADPH is the main source of reduction equivalents in RBCs, used by most of the protective systems. When NADPH becomes limiting, red cells are prone to being damaged. In many of the severe RBC enzyme deficiencies, a lack of protective enzyme activity is frustrating erythropoiesis or is not restricted to RBCs. Common hereditary RBC disorders, such as thalassemia, sickle-cell trait, and unstable hemoglobins, give rise to increased oxidative stress caused by free heme and iron generated from hemoglobin. The beneficial effect of thalassemia minor, sickle-cell trait, and glucose-6-phosphate dehydrogenase deficiency on survival of malaria infection may well be due to the shared feature of enhanced oxidative stress. This may inhibit parasite growth, enhance uptake of infected RBCs by spleen macrophages, and/or cause less cytoadherence of the infected cells to capillary endothelium.