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PubMed1970

The role of the cytoplasmic redox potential in the control of fatty acid synthesis from glucose, pyruvate and lactate in white adipose tissue.

The metabolism of lactate, pyruvate and glucose was studied in epididymal adipose tissue of starved, normally fed and starved-re-fed rats. Lactate conversion into fatty acid occurred at an appreciable rate only in the adipocyte of starved-re-fed animals. NNN'N'-Tetramethyl-p-phenylenediamine, an agent that transports reducing power from the cytoplasm to the mitochondria, caused large increments of fatty acid synthesis from lactate and a smaller one from glucose but a decrease in that from pyruvate. Glucose (1.0mm) increased fatty acid synthesis from lactate 4.3-fold but only 1.67-fold from pyruvate in adipocytes from normally fed animals. 2-Deoxyglucose decreased fatty acid synthesis from lactate to a greater degree (threefold) compared to that from pyruvate in adipocytes from starved-re-fed animals. l-Glycerol 3-phosphate contents were approximately equal in epididymal fat-pads, incubated in the presence of lactate or pyruvate, from normally fed animals, whereas the addition of 1mm-glucose resulted in a tenfold increase in l-glycerol 3-phosphate content only in the presence of lactate. The l-glycerol 3-phosphate content was tenfold higher in adipose tissue from starved-re-fed animals incubated in the presence of lactate than in the presence of pyruvate. 2-Deoxyglucose caused these values to be slightly lowered in the presence of lactate. We suggest that lactate metabolism is limited by the rate of NADH removal from the cytoplasm. In the starved-re-fed state, this occurs by reduction of dihydroxyacetone phosphate formed from glycogen to produce l-glycerol 3-phosphate, thus permitting lactate conversion into fatty acid. When glucose is the substrate, and rates of transport are not limiting, the rate of removal of cytoplasmic NADH limits glucose conversion into fatty acid.

Adipose Tissue↗
PubMed1986

Electron transfer to nitrogenase. Characterization of flavodoxin from Azotobacter chroococcum and comparison of its redox potentials with those of flavodoxins from Azotobacter vinelandii and Klebsiella pneumoniae (nifF-gene product).

Flavodoxin in the hydroquinone state acts as an electron donor to nitrogenase in several nitrogen-fixing organisms. The mid-point potentials for the oxidized-semiquinone and semiquinone-hydroquinone couples of flavodoxins isolated from facultative anaerobe Klebsiella pneumoniae (nifF-gene product, KpFld) and the obligate aerobe Azotobacter chroococcum (AcFld) were determined as a function of pH. The mid-point potentials of the semiquinone-hydroquinone couples of KpFld and AcFld are essentially independent of pH over the range pH 7-9, being -422 mV and -522 mV (normal hydrogen electrode) at pH 7.5 respectively. The mid-point potentials of the quinone-semiquinone couples at pH 7.5 are -200 mV (KpFld) and -133 mV (AcFld) with delta Em/pH of -65 +/- 4 mV (KpFld) and -55 +/- 2 mV (AcFld) over the range pH 7.0-9.5. This indicates that reduction of the quinone is coupled to protonation to yield a neutral semiquinone. The significance of these values with respect to electron transport to nitrogenase is discussed. The amino acid compositions, the N- and C-terminal amino acid sequences and the u.v.-visible spectra of KpFld and AcFld were determined and are compared with published data for flavodoxins isolated from Azotobacter vinelandii.

Amino Acid Sequence↗
PubMed2002

Role of redox potential of hemoglobin-based oxygen carriers on methemoglobin reduction by plasma components.

A functional requirement for all hemoglobin-based oxygen carriers (HBOCs) is the maintenance of the heme-iron in the reduced state. This is necessary for the reversible binding/release of molecular oxygen and minimization of methemoglobin (Fe+3) formation. Acellular hemoglobins are especially susceptible to oxidation and denaturation. In the absence of the intrinsic reducing systems of the red blood cell, the reduced heme-Fe+2 can be oxidized to form increasing levels of methemoglobin that can give rise to free radicals and oxidative cellular damage. If acellular HBOCs are to be utilized as red cell substitutes for oxygen delivery, these carriers must be stabilized in the plasma, the carrier medium. Normal plasma contains reducing components, such as ascorbic acid and glutathione, that can afford protection to these acellular HBOCs through electron-transfer mediated processes. For these components to provide effective reduction to an HBOC, a favorable reduction potential difference must exist between the reducing agent and the HBOC. Using a modified thin-layer spectroelectrochemical method, a determination of the formal reduction potential (vs. Ag/AgCl) of several oxygen carriers, including monomeric myoglobin, tetrameric HbA and HbS, chemically cross-linked HbXL99alpha, polymerized oxyglobin (FDA approved for canine anemia), and the natural cross-linked polymeric Lumbricus hemoglobin, have been determined. In contrast to the negative formal reduction potentials (-155 to -50 mV) obtained for Mb, HbA, HbS, HbXL99alpha, and oxyglobin, Lumbricus hemoglobin exhibited a positive formal reduction potential (approximately 100 mV). These results may help explain the greater effectiveness of the tested reducing agents to reduce met Lumbricus hemoglobin, compared to the other HBOCs, back to the required reduced form necessary for physiological binding/release of oxygen. Each reducing agent was capable of reducing met Lumbricus hemoglobin to the fully reduced state, although the kinetics of these reactions were different. HbA, HbXL99alpha, and oxyglobin were only partially reduced (10 to 37%) by glutathione, beta-NADH, and ascorbic acid under similar conditions.

Animals↗
PubMed2001

Substrates but not inhibitors alter the redox potentials of monoamine oxidases.

The midpoint potentials for the reduction of the cysteinyl-flavin adenine dinucleotide (FAD) in monoamine oxidases (MAO) A and B in the absence and presence of ligands have been determined. Both MAO A and MAO B can be reduced chemically in two steps, the first generating a semiquinone spectrum and the second the spectrum of fully reduced FAD, each of which requires two electron equivalents. The midpoint potentials for the oxidized/semiquinone and semiquinone/reduced couples were -159+/-4 mV and -262+/-3 mV for MAO A and -167+/-4 mV and -275+/-3 mV for MAO B. After modification with a thiol reagent, direct reduction from the oxidized to fully reduced form was observed with no semiquinone and without change in the overall midpoint potential. In the presence of substrate, no semiquinone was formed, but the midpoint potential for full reduction of the flavin was positively shifted by up to 500 mV, depending on the substrate. This shift in potential could permit a more thermodynamically favorable transfer of electrons from the amine substrates to oxygen. In contrast, stable products and inhibitors did not cause a shift in potential and did not prevent the formation of semiquinone.

Disulfides↗
PubMed1989

Relationship between the nicotinamide adenine dinucleotide redox potential and the 2,3-diphosphoglycerate content in the erythrocyte in sickle cell disease.

The percentage of nicotinamide adenine dinucleotide (NAD) in the oxidized form [NAD+/(NAD+ and NADH); i.e. the NAD+/NADT ratio] is increased in the red cell (RBC) in sickle cell disease. We tested the hypothesis that the increased NAD+/NADT ratio was a determinant of the increased 2,3-diphosphoglycerate (DPG) content of the SCD RBC. Using normal subjects and individuals with sickle cell disease or autoimmune haemolytic anaemia (AIHA), we observed an inverse relationship between the packed cell volume (PCV) and the RBC DPG concentration (r = -0.69) and a direct relationship between the RBC NAD+/NADT ratio and the DPG concentration (r = 0.74). When the effect of the PCV on DPG was removed using analysis of covariance [DPGady(PCV)], the NAD+/NADT ratio had a significant relationship with the DPGadj(PCV) (r = 0.50, P less than 0.001). In in vitro incubation studies, increasing the NAD+/NADT ratio significantly increased the DPG content of both normal and AIHA RBC. Conversely, decreasing the NAD+/NADT ratio decreased the DPG content of normal, AIHA and SCD RBC. Thus, the increased DPG content in the SCD RBC appears to be due, in part, to the increased NAD+/NADT ratio and is not purely a physiologic response to decreased oxygen carrying capacity.

2,3-Diphosphoglycerate↗