Effects of hemolysate concentration, ionic strength and cytochrome b5 concentration on the rate of methemoglobin reduction in hemolysates of human erythrocytes

Cytochrome b5: novel roles in steroidogenesis

Cytochrome b(5) (cyt-b(5)) is essential for the regulation of steroidogenesis and as such has been implicated in a number of clinical conditions. It is well documented that this small hemoprotein augments the 17,20-lyase activity of cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1). Studies have revealed that this augmentation is accomplished by cyt-b(5) enhancing the interaction between cytochrome P450 reductase (POR) and CYP17A1. In this paper we present evidence that cyt-b(5) induces a conformational change in CYP17A1, in addition to facilitating the interaction between CYP17A1 and POR. We also review the recently published finding that cyt-b(5) allosterically augments the activity of 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3βHSD), a non cytochrome P450 enzyme, by increasing the enzymes affinity for its cofactor, NAD(+). The physiological importance of this finding, in terms of understanding adrenal androstenedione production, is examined. Finally, evidence that cyt-b(5) is able to form homomeric complexes in living cells is presented and discussed.

Electron transfer between cytochrome b(5) and some oxidised haemoglobins: the role of ionic strength

We have compared experimental measurements and Brownian dynamic calculations for the reduction of oxidised adult human haemoglobin by reduced bovine cytochrome b(5) over a range of ionic strengths. Our calculations suggest that the presence of molecular electrostatic fields have a significant role to play in the formation of the electron transfer complexes. These results predict that electron transfer occurs within an ensemble of similarly weakly docked complexes, the formation of which is strongly ionic strength dependent. Application of electron tunneling analysis to the complexes allows us to predict the rates of electron transfer within each ensemble of complexes as a function of ionic strength. The outcome of this theoretical study is compared with experimental rate measurements. A comparison of the results obtained from adult and embryonic haemoglobins, at a fixed ionic strength, indicates a significant difference in the characteristics of complex formation. These data emphasise the role played by electrostatic interactions in this important physiological reaction.

The regulation of surface charged residues on the properties of cytochrome b5

To understand the roles of negatively surface charged residues, the cytochrome b5 (Cyt b5) E48A/D60A mutant was constructed. UV-visible and CD spectra confirmed that the mutation did not cause overall structural changes of the protein. The mutant presents an unexpected high stability toward the thermal and denaturant compared with the wild type Cyt b5, which shows that these surface charged residues can influence the interactions between the heme b group and the polypeptide chain. Functional properties were clarified through the electron transfer reactions between Cyt b5 and Cyt c. The driving force of the electron transfer reactions is conservative. Although the association constant of Cyt b5 E48A/D60A with Cyt c is much lower than that of the wild type Cyt b5, their electron transfer rate constants do not differ significantly. The results show that these surface charged residues play important roles in regulating both the stability and functional properties of Cyt b5.

Hemoglobin autooxidation/oxidation mechanisms and methemoglobin prevention or reduction processes in the bloodstream. Literature review and outline of autooxidation reaction

The amount of circulating methemoglobin in healthy humans is the result of a balance between methemoglobin production (from autooxidation and oxidation) and hemoglobin reduction. Hemoglobin autooxidation and oxidation are very complex and are not well understood. This article analyses the literature on hemoglobin autooxidation, oxidation and reduction and sets out a sequence of reactions for the oxidation of hemoglobin and the ways in which the percentage of methemoglobin is regulated or methemoglobin production prevented. Most of the information concerns erythrocyte hemoglobin, but plasma extracellular hemoglobin (from hemolysis or hemoglobin-based blood substitutes) is also considered where possible.

Cytochrome b5, its functions, structure and membrane topology

The first part of the present communication reviews recent advances in our understanding of the known physiological functions of cytochrome b5. In addition, one section is devoted to a description of a recently discovered function of cytochrome b5, namely its involvement in the synthesis of the oncofetal antigen N-glycolylneuraminic acid. The second part of the article summarizes site-directed mutagenesis studies, primarily conducted in the author's laboratory, in both the catalytic heme-binding and membrane-binding domain of cytochrome b5. These studies have shown that: 1) the membrane binding domain of cytochrome b5 spans the bilayer; 2) cytochrome b5 lacking 19 COOH-terminal amino acids does not bind to membrane bilayers; and 3) specific amino acids in the membrane binding domain have been mutated and shown not to be essential for the function of cytochrome b5 with its redox partners.

NMR characterization of surface interactions in the cytochrome b5-cytochrome c complex

The complex formed in solution by native and chemically modified cytochrome c with cytochrome b5 has been studied by 1H and 13C nuclear magnetic resonance spectroscopy (NMR). Contrary to predictions of recent theoretical analysis, 1H NMR spectroscopy indicates that there is no major movement of cytochrome c residue Phe82 on binding to cytochrome b5. The greater resolution provided by 13C NMR spectroscopy permits detection of small perturbations in the environments of cytochrome c residues Ile75 and Ile85 on binding with cytochrome b5, a result that is in agreement with earlier model-building experiments. As individual cytochrome c lysyl residues are resolved in the 1H NMR spectrum of N-acetimidylated cytochrome c, the interaction of this modified protein with cytochrome b5 has been studied to evaluate the number of cytochrome c lysyl residues involved in binding to cytochrome b5. The results of this experiment indicate that at least six lysyl residues are involved, two more than predicted by static model building, which indicates that cytochrome c and cytochrome b5 form two or more structurally similar 1:1 complexes in solution.

Cytochrome b5 and NADH-cytochrome-b5 reductase from sipunculan erythrocytes; a methemerythrin reduction system from Phascolopsis gouldii

We report the purification and characterization of a soluble cytochrome b5 from coelomic erythrocytes of the sipunculan worm, Phascolopsis gouldii. We also report the isolation and purification of a membrane-bound NADH-cytochrome-b5 reductase from these erythrocytes. The non-heme iron protein, hemerythrin (Hr), is known to be the oxygen carrier in these erythrocytes. The aforementioned purified cytochrome b5 and reductase together catalyze the reduction of P. gouldii [Fe(III),Fe(III)]metHr to [Fe(II),Fe(II)deoxyHr by NADH. EPR spectroscopy demonstrates that a redox process involving formation of the intermediate [Fe(II),Fe(III)]semi-metHr occurs within intact sipunculan erythrocytes as well as in the system of purified components. The rhombic g-tensor of the EPR signal in both cases resembles that of (semi-met)RHr, the form obtained by one-electron reduction of metHr. These observations suggest that cytochrome b5 and NADH-cytochrome-b5 reductase in sipunculan erythrocytes function to counteract autoxidation of oxyHr. The sequence of electron flow in the system of purified components is: NADH----NADH-cytochrome-b5 reductase----cytochrome b5----metHr. At pH 7.5, the reduction of metHr in this system occurs in two phases, only the first of which is dependent on concentration of cytochrome b5. From an analysis of the kinetics and the EPR time-course, we propose that the two phases represent sequential reduction of met- to semi-metHr and reduction of semi-metHr to deoxyHr. This report represents the first demonstration of a physiological system for reduction of metHr.

Studies on the energy metabolism of opossum Didelphis virginiana erythrocytes--III. Metabolic depletion with 2-deoxyglucose markedly accelerates methemoglobin reduction in opossum but not in human erythrocytes

1. Glucose-depleted, nitrite-treated erythrocytes reduce ferriheme in vitro in an environment 100 mM to 2-deoxy-D-glucose at a rate of 2.4 microM/ml cells/hr (opossum) and 0.37 microM/ml cells/hr (human). 2. During the process of methemoglobin reduction the breakdown of adenine ribonucleotides is more rapid in opossum (0.9 microM/g hg/hr) than in human (0.36 microM/g hg/hr) erythrocytes. 3. Radiolabelled ribose from [U-14C] ATP is catabolized exclusively to [14C] lactate in opossum, and to [14C] pyruvate and [14C] lactate in human red cells.

Conversion of oxyhaemoglobin into methaemoglobin by ferricytochrome b5

Ferricytochrome b5 was found to convert oxyhaemoglobin into methaemoglobin under conditions previously found to be optimal for complex-formation between ferricytochrome b5 and methaemoglobin [Mauk & Mauk (1982) Biochemistry 21, 4730-4734]. As this reaction is completely inhibited by CO, it is proposed that oxyhaemoglobin is oxidized after O2 dissociation, as has been suggested for the oxidation of oxyhaemoglobin by inorganic complexes. From the present analysis, ferricytochrome b5 seems unlikely to contribute significantly to methaemoglobin formation in vivo. Nevertheless, this observation provides a relatively convenient means of investigating the mechanism by which these two proteins interact.

Ferric leghemoglobin reductase from soybean root nodules

An NADH: (acceptor) oxidoreductase from the cytosol of soybean root nodules was purified by ammonium sulfate fractionation, hydroxylapatite adsorption, and Sephacryl S-200 Superfine chromatography. The native molecular weight of the reductase was found to be 100,000 by analytical gel filtration and 83,000 by equilibrium ultracentrifugation. The subunit molecular weight was 54,000 as determined by sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis. The pI of the enzyme was 5.5. With ferric leghemoglobin (Lb) as the substrate, nearly identical initial velocities were obtained using either CO or O2 to ligate the enzymatically produced ferrous leghemoglobin. With CO as the ligand in the reaction, the product of the enzyme-catalyzed, NADH-dependent reduction of ferric Lb was spectrally identified as LbCO. Initial velocity was a linear function of increasing enzyme concentration. NADPH was only 31% as effective an electron donor as NADH as determined by initial velocity. The Michaelis constants (Km) for ferric Lba and NADH were 9.5 and 18.8 microM, respectively. Myoglobin, Lba, Lbc1, Lbc2, Lbc3, and Lbd were reduced at similar rates by the reductase. At pH 5.2, acetate-bound ferric Lb and nicotinate-bound ferric Lb were reduced by the enzyme at 83 and 5%, respectively, of rates observed in the absence of these ligands. The rate of enzymatic reduction of ferric Lb was constant between pH 6.5 and 7.6 but increased approximately threefold at pH 5.2. The results indicate that the NADH: (acceptor) oxidoreductase could be identified as a ferric Lb reductase.

Primaquine-mediated oxidative metabolism in the human red cell. Lack of dependence on oxyhemoglobin, H2O2 formation, or glutathione turnover

Stimulation of the hexose monophosphate shunt by primaquine results from the oxidation of NADPH by primaquine. This conclusion was based on the observations that primaquine lowered cellular NADPH but not GSH and that, in red cells in which the GSH was unavailable for reaction, primaquine still stimulated the rate of the hexose monophosphate shunt. In a non-cellular system, primaquine interacted with NADPH, but not GSH, to produce H2O2. Stimulation of the hexose monophosphate shunt by primaquine does not primarily involve H2O2 accumulation since stimulation of the pathway by primaquine was also observed in red cells containing methemoglobin, a red cell preparation in which no H2O2 accumulates. Methemoglobin prevented the formation and/or accumulation of H2O2 in intact red cells incubated with primaquine as well as in a non-cellular system containing primaquine plus Fe2+-EDTA as an H2O2 source. Methemoglobin probably acts by scavenging reactive intermediates since oxyhemoglobin was formed from methemoglobin in the non-cellular experiments. In the red cell, primaquine stimulated glucose-dependent conversion of methemoglobin to oxyhemoglobin.

Age-dependent decay of cytochrome b5 and cytochrome b5 reductase in human erythrocytes

Age-dependent decrease in cytochrome b5 was observed in erythrocytes from both a normal person and a patient with hereditary methaemoglobinaemia without neurological symptoms. With aging, concentrations of cytochrome b5 in erythrocytes from the patient were almost the same as those in the control. Age-dependent decrease in cytochrome b5 reductase activity in the control erythrocytes was also shown; however, the reductase activity was very low in erythrocytes from the patient over the whole age range. Our studies show that methaemoglobin content of erythrocytes seems to be dependent on the content of cytochrome b5 in the cells, both in the control subject and in the patient.