The role of the flap residue, threonine 77, in the activation and catalytic activity of pepsin A.
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Flexible loops, often referred to as flaps, have been shown to play a role in catalytic mechanisms of different enzymes. Flaps at the active site regions have been observed in the crystal structures of aspartic proteinases and their residues implicated in the catalytic processes. This research investigated the role of the flap residue, threonine 77, in the activation of pepsinogen and the catalytic mechanism of pepsin. Three mutants, T77S, T77V and T77G, were constructed. Differences in amino acid polarity and hydrogen bonding potential were shown to have an influence on the activation and catalytic processes. T77S activated at the same rate and had similar catalytic parameters as the wild-type pepsin. The activation rates of T77V and T77G were slower and their catalytic efficiencies lower than the wild-type. The results demonstrated that the threonine 77 polar side chain played a role in a proteolysis. The contribution of the side chain to zymogen activation was associated with the proteolytic cleavage of the prosegment. It was postulated that the hydroxyl group at position 77 provided an essential hydrogen bond that contributed to proper substrate alignment and, indirectly, to a catalytically favorable geometry of the transition state. (+info)
Association and dissociation kinetics of bobwhite quail lysozyme with monoclonal antibody HyHEL-5.
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The anti-hen egg lysozyme monoclonal antibody HyHEL-5 and its complexes with various species-variant and mutant lysozymes have been the subject of considerable experimental and theoretical investigation. The affinity of HyHEL-5 for bobwhite quail lysozyme (BWQL) is over 1000-fold lower than its affinity for the original antigen, hen egg lysozyme (HEL). This difference is believed to arise almost entirely from the replacement in BWQL of the structural and energetic epitope residue Arg68 by lysine. In this study, the association and dissociation kinetics of BWQL with HyHEL-5 were investigated under a variety of conditions and compared with previous results for HEL. HyHEL-5-BWQL association follows a bimolecular mechanism and the dissociation of the antibody-antigen complex is a first-order process. Changes in ionic strength (from 27 to 500 mM) and pH (from 6.0 to 10.0) produced about a 2-fold change in the association and dissociation rates. The effect of viscosity modifiers on the association reaction was also studied. The large difference in the HEL and BWQL affinities for HyHEL-5 is essentially due to differences in the dissociation rate constant. (+info)
Regulation of myocardial blood flow by oxygen consumption is maintained in the failing heart during exercise.
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The hemodynamic abnormalities and neurohumoral activation that accompany congestive heart failure (CHF) might be expected to impair the increase in coronary blood flow that occurs during exercise. This study was performed to determine the effects of CHF on myocardial oxygen consumption and coronary blood flow during exercise. Coronary blood flow was measured in chronically instrumented dogs at rest, during 2 stages of graded treadmill exercise under control conditions (n=10), and after the development of CHF produced by 3 weeks of rapid ventricular pacing (n=9). In the normal dogs, coronary blood flow increased during exercise in proportion to the increase in the heart rate x the left ventricular systolic blood pressure product (RPP). After the development of CHF, resting myocardial blood flow was 25% lower than normal (P<0.05). Myocardial blood flow increased during the first stage of exercise, but then failed to increase further during the second stage of exercise despite an additional increase in the RPP. Myocardial oxygen consumption during exercise was significantly lower in animals with CHF and paralleled coronary flow. Despite the lower values for coronary blood flow in animals with CHF, there was no evidence for myocardial ischemia. Thus, even during the second level of exercise when coronary flow failed to increase, myocardial lactate consumption continued and coronary venous pH did not fall. In addition, the failure of coronary flow to increase as the exercise level was increased from stage 1 to stage 2 was not associated with a further increase in myocardial oxygen extraction. Thus, cardiac failure was associated with decreased myocardial oxygen consumption and failure of oxygen consumption to increase with an increase in the level of exercise. This abnormality did not appear to result from inadequate oxygen availability, but more likely represented a reduction of myocardial oxygen usage with a secondary decrease in metabolic coronary vasodilation. (+info)
Evidence that neuroepithelial endocrine cells control the spontaneous tone in guinea pig tracheal preparations.
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The hypothesis that neuroepithelial endocrine (NEE) cells control spontaneous tone in isolated guinea pig tracheal preparations was examined. Epithelium-denuded preparations were unable to develop a normal oscillating tone in 12% oxygen (corresponding to systemic arterial oxygen levels) and, instead, developed a strong, smooth tone, similar to the "classic" tone in 94% oxygen. Inhibition of the hydrogen peroxide-producing NADPH oxidase in the NEE cells by 20 microM diphenyleneiodonium chloride transformed, in intact preparations in 94% oxygen, the tone from a strong, smooth type to an oscillating tone of considerably less force. Similar experiments in denuded preparations showed no change of tone and no oscillations. After pretreatment with the catalase inhibitor 3-amino-1,2, 4-triazole (1 mM), addition of 2 mM hydrogen peroxide to intact preparations displaying the oscillating tone caused a transformation to a strong, smooth type. These findings support the hypothesis that the spontaneous tone in this preparation is largely controlled by the oxygen-sensing NEE cells. For the first time, previous findings on isolated cells can be linked to effects in intact tissue preparations. The results also suggest that the regulation by the NEE cells involves the release of powerful relaxing and contracting factors from the epithelium. (+info)
Efficacy of recombinant human Hb by 31P-NMR during isovolemic total exchange transfusion.
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The ability of recombinant human Hb (rHb1.1), which is being developed as an oxygen therapeutic, to support metabolism was measured by in vivo 31P-NMR surface coil spectroscopy of the rat abdomen in control animals and in animals subjected to isovolemic exchange transfusion to hematocrit of <3% with human serum albumin or 5 g/dl rHb1.1. No significant changes in metabolite levels were observed in control animals for up to 6 h. The albumin-exchange experiments, however, resulted in a more than eightfold increase in Pi and a 50% drop in phosphocreatine and ATP within 40 min. The tissue pH dropped from 7.4 to 6.8. The decrease in high-energy phosphates obeyed Michaelis-Menten kinetics, with a Michaelis-Menten constant of 3% as the hematocrit at which a 50% drop in high-energy phosphates was observed. Exchange transfusion with rHb1.1 resulted in no significant drop in high-energy phosphates, no rise in Pi, and no change in tissue pH from 7.35 +/- 0.15 for up to 5 h after exchange. By these criteria, rHb1.1 at a plasma Hb concentration of approximately 5 g/dl after total exchange transfusion was able to sustain energy metabolism of gut tissue at levels indistinguishable from control rats with a threefold higher total Hb level in erythrocytes. (+info)
Chlamydomonas chloroplast ferrous hemoglobin. Heme pocket structure and reactions with ligands.
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We report the optical and resonance Raman spectral characterization of ferrous recombinant Chlamydomonas LI637 hemoglobin. We show that it is present in three pH-dependent equilibrium forms including a 4-coordinate species at acid pH, a 5-coordinate high spin species at neutral pH, and a 6-coordinate low spin species at alkaline pH. The proximal ligand to the heme is the imidazole group of a histidine. Kinetics of the reactions with ligands were determined by stopped-flow spectroscopy. At alkaline pH, combination with oxygen, nitric oxide, and carbon monoxide displays a kinetic behavior that is interpreted as being rate-limited by conversion of the 6-coordinate form to a reactive 5-coordinate form. At neutral pH, combination rates of the 5-coordinate form with oxygen and carbon monoxide were much faster (>10(7) microM-1 s-1). The dissociation rate constant measured for oxygen is among the slowest known, 0.014 s-1, and is independent of pH. Replacement of the tyrosine 63 (B10) by leucine or of the putative distal glutamine by glycine increases the dissociation rate constant 70- and 30-fold and increases the rate of autoxidation 20- and 90-fold, respectively. These results are consistent with at least two hydrogen bonds stabilizing the bound oxygen molecule, one from tyrosine B10 and the other from the distal glutamine. In addition, the high frequency (232 cm-1) of the iron-histidine bond suggests a structure that lacks any proximal strain thus contributing to high ligand affinity. (+info)
Adenosine inhibits the transfected Na+-H+ exchanger NHE3 in Xenopus laevis renal epithelial cells (A6/C1).
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1. Adenosine influences the vectorial transport of Na+ and HCO3- across kidney epithelial cells. However, its action on effector proteins, such as the Na+-H+ exchanger NHE3, an epithelial brush border isoform of the Na+-H+ exchanger (NHE) gene family, is not yet defined. 2. The present study was conducted in Xenopus laevis distal nephron A6 epithelia which express both an apical adenosine receptor of the A1 type (coupled to protein kinase C (PKC)) and a basolateral receptor of the A2 type (coupled to protein kinase A (PKA)). The untransfected A6 cell line expresses a single NHE type (XNHE) which is restricted to the basolateral membrane and which is activated by PKA. 3. A6 cell lines were generated which express exogenous rat NHE3. Measurements of side-specific pHi recovery from acid loads in the presence of HOE694 (an inhibitor with differential potency towards individual NHE isoforms) detected an apical resistant Na+-H+ exchange only in transfected cell lines. The sensitivity of the basolateral NHE to HOE694 was unchanged, suggesting that exogenous NHE3 was restricted to the apical membrane. 4. Stimulation of the apical A1 receptor with N 6-cyclopentyladenosine (CPA) inhibited both apical NHE3 and basolateral XNHE. These effects were mimicked by the addition of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) and partially prevented by the PKC inhibitor calphostin C which also blocked the effect of PMA. 5. Stimulation of the basolateral A2 receptor with CPA inhibited apical NHE3 and stimulated basolateral XNHE. These effects were mimicked by 8-bromo-cAMP and partially prevented by the PKA inhibitor H89 which entirely blocked the effect of 8-bromo-cAMP. 6. In conclusion, CPA inhibits rat NHE3 expressed apically in A6 epithelia via both the apical PKC-coupled A1 and the basolateral PKA-coupled A2 adenosine receptors. (+info)
A novel role for carbonic anhydrase: cytoplasmic pH gradient dissipation in mouse small intestinal enterocytes.
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1. The spatial and temporal distribution of intracellular H+ ions in response to activation of a proton-coupled dipeptide transporter localized at the apical pole of mouse small intestinal isolated enterocytes was investigated using intracellular carboxy-SNARF-1 fluorescence in combination with whole-cell microspectrofluorimetry or confocal microscopy. 2. In Hepes-buffered Tyrode solution, application of the dipeptide Phe-Ala (10 mM) to a single enterocyte reduced pHi locally in the apical submembranous space. After a short delay (8 s), a fall of pHi occurred more slowly at the basal pole. 3. In the presence of CO2/HCO3--buffered Tyrode solution, the apical and basal rates of acidification were not significantly different and the time delay was reduced to 1 s or less. 4. Following application of the carbonic anhydrase inhibitor acetazolamide (100 microM) in the presence of CO2/HCO3- buffer, addition of Phe-Ala once again produced a localized apical acidification that took 5 s to reach the basal pole. Basal acidification was slower than at the apical pole. 5. We conclude that acid influx due to proton-coupled dipeptide transport can lead to intracellular pH gradients and that intracellular carbonic anhydrase activity, by facilitating cytoplasmic H+ mobility, limits their magnitude and duration. (+info)