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71.
Metabolic acidosis produces a phosphaturia which is independent of parathyroid hormone or dietary phosphorus intake. To study the underlying mechanism, inorganic phosphate (Pi) and glucose transport were studied in brush-border membrane vesicles prepared from the renal cortex of parathyroidectomized rats gavaged for three days with either 7.5 ml of 1.6% NaCl (control) or 1.5% NH4Cl (acidosis). At killing, blood pH and plasma bicarbonate were and , respectively, in control and () and () in acidotic rats. Serum Pi was similar in both groups, while 24 h urine Pi excretion was higher in the acidotic group (). Peak sodium-dependent uptake of Pi, measured after 1.5 min of incubation, was higher in controls than acidotic rats ( vs. protein, ), whereas peak glucose uptake at 1.5 min was not significantly different between the groups. Equilibrium values for Pi and glucose uptake were similar in the two groups. for Pi uptake in the control and acidotic animals were not different, 0.036 and 0.040 mM, respectively. By contrast, was higher in controls than in the acidotic group, 3.13 vs. 1.15 nmol/mg protein per 15 s. These results suggest that metabolic acidosis directly inhibits Pi uptake by the brush border of the proximal tubule by decreasing the availability of Pi carriers of the renal brush-border membrane. 相似文献
72.
Gianfranco Menestrina 《生物化学与生物物理学报:生物膜》1983,735(2):297-301
Megathura crenulata hemocyanin forms ionic channels in planar lipid bilayer membranes. It was found that hemocyanin is more potent as a channel former if TbCl3 is added to the bathing solution. Furthermore membranes separating symmetrical TbCl3 solutions show a pore formation rate which depends exponentially on the applied voltage, positive potentials favouring the insertion of new channels. The slope of this voltage dependence, which gives a measure of the effective charge displaced during the incorporation of one channel, increases and saturates with TbCl3 concentration. The dose response curve indicates that binding of Tb3+ to the phosphatidylcholine bilayer is involved in creating the effective charge. 相似文献
73.
The acid hydrolysis product of saxitoxin is shown to be decarbamoylsaxitoxin by spectral characterization and its reconversion to saxitoxin by carbamoylation. Natural and resynthesized saxitoxin are identical in chromatographic and spectral properties and in their potencies in blocking the sodium channel in squid giant axon. The hydrolysis product, decarbamoylsaxitoxin, exhibits 20% of the potency of saxitoxin in the squid axon system. These results confirm the structure of the hydrolysis product and its biological activity relative to saxitoxin. 相似文献
74.
Comparison of six rabbit liver cytochrome P-450 isozymes in formation of a reactive metabolite of acetaminophen 总被引:7,自引:0,他引:7
This laboratory has recently reported the isolation of an ethanol-inducible form of rabbit liver microsomal cytochrome P-450, designated isozyme 3a. In view of the reports of others that the hepatotoxicity of acetaminophen is increased in ethanol-treated animals and the human alcoholic, we have determined the activity of the six available P-450 isozymes in the activation of the drug to give an intermediate which forms a conjugate with reduced glutathione. Isozymes 3a, 4, and 6, all of which are present in significant amounts in the liver microsomes from rabbits chronically administered ethanol, exhibited the highest activities in the reconstituted enzyme system, whereas isozymes 3b and 3c were 10- to 20-fold less effective, and phenobarbital-inducible isozyme 2 was essentially inactive, even in the presence of cytochrome b5. The results obtained thus indicate that induction by ethanol of P-450 isozyme 3a (or a homologous enzyme in other species) may contribute to the toxicity of acetaminophen but that other cytochromes also play a significant role. 相似文献
75.
M.Motasim Billah John M. Johnston 《Biochemical and biophysical research communications》1983,113(1):51-58
Human amniotic fluid and fetal urine were examined for the presence of phospholipid platelet-activating factor (PAF). PAF was detected in lipid extracts of some samples of amniotic fluid obtained from women in labor but it was undetectable in samples of amniotic fluid obtained before the onset of labor. PAF was identified by chromatographic mobility, platelet aggregation and chemical modifications. LysoPAF was also present in amniotic fluid at higher concentrations than those of PAF. Both PAF and lysoPAF were identified also in newborn and adult urine. 相似文献
76.
The interaction between glucagon and dicaprylphosphatidylcholine (DCPC) was studied by fluorescence, circular dichroism and calorimetry, as well as by 1H- and 31P-nuclear magnetic resonance. The water-soluble lipid-protein complex was also characterized by gel filtration and ultracentrifugation. The complex appeared to be monodisperse by sedimentation equilibrium measurements, with a molecular weight of (4.55 ± 0.57)·104. This complex contained approximately 7 molecules of glucagon and 35 molecules of phospholipid. Proton-decoupled 31P-NMR spectra of the phospholipid in the lipid-protein complex display narrower resonances than those of sonicated vesicles of DCPC, and 1H-31P coupling could be detected in proton coupled spectra. These NMR results, together with gel-filtration results, suggest that glucagon ‘solubilizes’ phospholipid aggregates, forming a lipid-protein complex which is smaller than sonicated preparations of DCPC. 1H-NMR resonance of both the methionine methyl group (met-27) and the aromatic envelope of glucagon are broadened by the phospolipid, indicating that the C-terminal region and the aromatic residues are involved in the interaction with the phospholipid. Nuclear magnetic resonance titrations of the imidazole ring C(2) and C(4) protons of the histidine residue of glucagon show that DCPC lowers the pK of the imidazole. The alterations caused by the phospholipid in the far and near ultraviolet CD spectra of glucagon reflect, respectively, the increased helix content of the hormone and the fact that the aromatic residues are located in a more structured environment. The phospholipid also alters the fluorescence properties of glucagon, shifting the fluorescence emission maximum of the hormone to shorter wavelength, and enhancing its relative intensity. This suggests that the fluorophore is experiencing a more hydrophobic environment in the presence of the lipid. Binding of glucagon to the phospholipid was analysed by Scatchard plots of the enhancement of fluorescence caused by the phospholipid and showed that the equilibrium binding constants of glucagon to DCPC are (4.4 ± 0.5)·104M?1 and (7.5±0.5)·104M?1, at 15°C and 25°C, respectively. The average number of moles of phospholipid bound per mole of glucagon is 4.4±0.6. The isothermal enthalpy of reaction of glucagon with DCPC is ?20.5 kcal/mol of glucagon at 25°C and ?32.5 kcal/mol of glucagon at 15°C. The observed enthalpies can arise from glucagon-induced cyrstallization of the phospholipid, from the non-covalent interactions between the peptide and lipid as well as from the lipid-induced conformational change in the protein. These results demonstrate that, unlike the complexes formed between glucagon and phospholipids which form more stable bilayers, the complex formed between glucagon and DCPC is stable over a wide range of temperatures, including temperatures well above the phase transition. 相似文献
77.
The interaction between the oxidized forms of cytochrome c and cytochrome c oxidase (EC 1.9.3.1) has been investigated by 1H-NMR longitudinal relaxation measurements. It is found that relaxation of methyl groups on the heme ring of cytochrome c markedly deviates from a simple exponential behavior in the presence of small amounts of cytochrome oxidase. A comparison with the relaxation behavior of cytochrome c modified by 4-carboxy-3,5-dinitrophenyl at Lys-13 shows that the oxidase induces a conformation in native cytochrome c that is closely related to that of the derivative. It is suggested that this change in conformation consists of a rupture of the salt bridge between Lys-13 and Glu-90 and a concomitant perturbation of the methionine ligand. 相似文献
78.
(1) Using asolectin (mixed soybean phospholipids) liposomes, extra lipid, with or without additional plastoquinone, has been introduced into isolated thylakoid membranes of pea chloroplasts. (2) Evidence for this lipid enrichment was obtained from freeze-fracture which indicated that a decrease in the numbers of EF and PF particles per unit area of membrane occurred with increasing lipid incorporation. The decrease was not due to loss of integral membrane polypeptides as judged by assay of cytochrome present or SDS-polyacrylamide gel electrophoresis of lipid-enriched membrane fractions. Moreover, the enrichment procedure did not lead to extraction of low molecular weight lipophilic membrane components or of thylakoid membrane lipids. (3) The introduction of phospholipids into the membrane affected steady-state electron transport. Inhibition of electron transport was observed when either water (Photosystem (PS) II + PS I) or duroquinol (PS I) was used as electron donor with methyl viologen as electron acceptor, and the degree of inhibition increased with higher enrichment levels. Introduction of exogenous plastoquinone with the additional lipid had little effect on whole-chain electron transport, but caused an increase in the 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB)-sensitive rate of PS I electron transport. The inhibition was also detected by flash-induced oxidation-reduction changes of cytochrome f. 相似文献
79.
The hypothesis that chloroplasts having different light-saturated rates of photosynthesis will have different proportions of the intrinsic thylakoid complexes engaged in light-harvesting and electron transport (Anderson, J.M. (1982) Mol. Cell. Biochem. 46, 161–172) has been tested. Peas were grown in light regimes which varied in light intensity, quality and time of irradiance, and ranged from sunlight through red to blue-enriched light of very low radiation. The electron-transport capacity at saturating light of Photosystem I and Photosystem II of chloroplasts isolated from light-adapted peas was 2-fold and 5–6-fold lower, respectively, in the lowest radiation compared to sunlight. There was a marked increase in the amount of total chlorophyll associated with the main chlorophyll (LHCP1, LHCP2 and LHCP3) and a 2-fold decrease in the core reaction centre complex of Photosystem II (CP a) as the radiation decreased; the a ratio changed from 3.5 to 9.0. The amount of chlorophyll associated with Photosystem I varied from 34% in sunlight to 27% in the lowest radiation, but the antenna size of Photosystem I was not markedly different; there was a 2-fold decrease in the amount of cytochrome f on a chlorophyll basis, which partly accounted for the decreased electron-transport capacity of Photosystem I. Since the increases or decreases in the levels of each of the components correlated with decreasing radiation, it is clear that the light-adaptation of both light-harvesting and electron-transport components is indeed closely co-ordinated. 相似文献
80.
Different cross-linkers (10 mM) of varying specificity and arm length were found to cross-link mitochondrial matrix proteins in situ in 2 min at pH 7.4. As seen by SDS-polyacrylamide electrophoresis, the disappearance of individual protein bands was accompanied by concomitant appearance of polymeric aggregates that failed to enter the 4% spacer gel. The disorganization of the mitochondrial matrix infrastructure either by swelling or sonication of the mitochondria resulted in a decrease in the rate of cross-linking. Leakage of citrate synthase, malate dehydrogenase and fumarase was found to be reduced when cross-linked mitochondria were made permeable with toluene. On lysing the cross-linked mitochondria, a major part of the matrix protein (75%) was found to sediment with the membrane fraction. The activities of citrate synthase, malate dehydrogenase and fumarase in rat liver mitochondria were also found to increase in the precipitates with a concomitant decrease in their activities in the soluble matrix fraction. These results indicate that the cross-linker enters the mitochondria and cross-links matrix proteins including Krebs cycle enzymes either to the mitochondrial membranes, or to themselves resulting in very large molecular weight complexes. These results are interpreted to mean that in liver mitochondria, the Krebs cycle enzymes are preferentially located near the membrane. 相似文献