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Biomedical subjects

S I Chan

Publications and source records attributed to S I Chan.

At least 127 records · Page 7Linked to original sources
PubMed1981

Nuclear magnetic resonance studies of cation transport across vesicle bilayer membranes.

We analyze an increasingly popular NMR method analogous to the black lipid membrane (BLM) isotopic tracer experiment for the study of mediated cation transport but involving the preparation of vesicles with an environment asymmetric in that paramagnetic metal ions are present only outside the vesicles. This asymmetry is manifest in the NMR spectrum as two distinct resonances for magnetic nuclei in outside and inside lipid headgroups. As mediated transport begins and for the paramagnetic metal ions enter the vesicles, the inner headgroup resonance line shifts and changes shape with a time course containing much information on the actual ion transport mechanism. Processes by which the ions enter the vesicles one or a few at a time (such as via a diffusive carrier) are easily distinguishable from those by which the ions enter in large bursts (such as by pore activation). The limiting case where intervesicular mediator exchange is slow relative to cation transport (the situation for integral membrane proteins) is treated analytically. Computer simulated curves indicate conditions necessary for certain changes in the line shape which are analogous to the "current jumps" observed in BLM conductance studies. The theory derived allows estimates of the average number of ions entering the first few bursts, how often the bursts occur, and how they depend on the concentration of the mediating species in the vesicular membrane. Preliminary experimental spectra illustrating some of the various possible line shape behaviors are presented.

Cations↗
PubMed1981

Huntington disease and Tourette syndrome. I. Electron spin resonance of bed ghosts.

Abnormalities in the electron spin resonance (ESR) of nitroxide-labeled red blood cell membranes have been reported in Huntington disease (HD). Because of the importance of verifying a general membrane defect in this disease, we have examined 13 unmedicated HD patients of all grades of severity, with a duration of symptoms from 3 to 20 years and including juvenile and rigid cases. No significance differences in the ESR of controls, HD patients, and three Tourette syndrome patients were found.

Adolescent↗
PubMed1980

Reactions of nitric oxide with cytochrome c oxidase.

The reactions of nitric oxide (NO) with both oxidized and reduced cytochrome c oxidase are reported. NMR and mass spectroscopy were utilized to determine the products of the reactions; EPR and optical spectroscopy were employed to determine the states of the enzyme produced in each of these reactions. It was found that the enzyme catalyzes the consecutive oxidation and reduction of NO. A different cycle was observed when NO was added to the reduced enzyme, to the oxidized enzyme, or to the oxidized enzyme in the presence of azide. It was possible to observe the state of the enzyme at several points in each of these three cycles by varying the concentration of NO. The reactions of NO all involved a one- or two-electron redox step and could be accounted for by the involvement of only cytochrome a3 and Cua3. On the basis of these results, a mechanism for the reduction of dioxygen by the enzyme is proposed in which cytochrome a3 functions to anchor dioxygen and intermediates while remaining in the ferrous state, whereas Cua3 functions to accept electroins from cytochrome a/Cua and transfer them to dioxygen.

Binding Sites↗
PubMed1980

Evidence for the absence of photoreduction of the metal centers of cytochrome C oxidase by X-irradiation.

Samples of X-irradiated cytochrome c oxidase were examined by electron paramagnetic resonance and optical spectroscopy. Both radiation from the Stanford Synchrotron Radiation Laboratory and a conventional X-ray source (W target) were utilized. The X-ray flux from these sources ranges from 10(9) to 10(13) photons/s. No evidence was found for photoreduction of the metal centers in the enzyme by X-ray photons. These results demonstrate that the integrity of cytochrome c oxidase is maintained using the conditions under which X-ray absorption measurements are presently being made.

Animals↗
PubMed1980

The effect of surface curvature on the head-group structure and phase transition properties of phospholipid bilayer vesicles.

Proton nuclear magnetic resonance spectra at 360 MHz of small sonicated distearoyl phosphatidylcholine vesicles show easily distinguishable resonances due to choline N-methyl head-group protons located in the inner and outer bilayer halves. A study of the chemical shift of these resonances as a function of temperature reveals that the splitting between them increases below the phase transition. This occurs as a result of an upfield shift of the inner layer resonance at the phase transition. Consideration of the possible causes of this effect results in the conclusion that, at the phase transition, there is a change in the organization of the inner layer head-groups which does not occur for the outer layer head-groups.

Choline↗
PubMed1980

Lipoprotein uptake in perfused arteries: inhibition by 7-ketocholesterol.

The effect of 7-Ketocholesterol (7-KC), an oxygenated sterol (OS) on the uptake of low density lipoproteins (LDL) by artificially perfused porcine carotid arteries was studied. The perfusate consisted of Dulbecco's modified Eagle medium, supplemented with albumin and pure 125I-LDL. Prior exposure to 7-KC significantly inhibited LDL uptake (from 2.47 to 1.92 micrograms LDL p/gr artery). The degree of inhibition of LDL uptake was less than that previously observed for arterial influx of cholesterol.

Animals↗
PubMed1979

Resonance Raman spectra of cytochrome c oxidase. Excitation in the 600-nm region.

The resonance Raman (RR) spectra of oxidized, reduced, and oxidized cyanide-bound cytochrome c oxidase with excitation at several wavelengths in the 600-nm region are presented. No evidence is found for laser-induced photoreduction of the oxidized protein with irradiation at lambda approximately 600 nm at 195 K, in contrast to the predominance of this process upon irradiation in the Soret region at this temperature. The Raman spectra of all three protein species are very similar, and there are no Raman bands which are readily assignable to either cytochrome a or cytochrome a3 exclusively. The Raman spectra of the three protein species do, however, exhibit a number of bands not observed in the RR spectra of other hemoproteins upon exicitation in their visible absorption bands. In particular, strong Raman bands are observed in the low-frequency region of the RR spectra (less than 500 cm-1). The frequencies of these bands are similar to those of the copper-ligand vibrations observed in the RR spectra of type 1 copper proteins upon excitation in the 600-nm absorption band characteristic of these proteins. In cytochrome c oxidase, these bands do not disappear upon reduction of the protein and, therefore, cannot be attributed to copper-ligand vibrations. Thus, all the observed RR bands are associated with the two heme A moieties in the enzyme.

Animals↗
PubMed1979

Physicochemical characterization of 1,2-diphytanoyl-sn-glycero-3-phosphocholine in model membrane systems.

We report here on a series of studies aimed at characterization of the structural and dynamical properties of the synthetic lipid diphytanoyl phosphatidylcholine, in multilamellar dispersions and vesicle suspensions. The lipid exhibits no detectable gel to liquid crystalline phase transition over a large temperature range (-120 degrees C to +120 degrees C). Examination of proton nuclear magnetic resonance (NMR) free induction decays obtained from multilayer dispersions of diphytanoyl phosphatidylcholine provided an estimate of the methylene proton order parameter. The estimated magnitude of 0.21 is comparable to those determined for other phospholipids. Sonication of aqueous dispersions of diphytanoyl phosphatidylcholine led to formation of bilayer vesicles as determined by the measurement of the outer/inner choline methyl proton resonances, vesicle sizes in electron micrographs, and comparison of proton NMR linewidths between multilayer and sonicated dispersions. Ultracentrifugation studies of diphytanoyl phosphatidylcholine vesicles in H2O and 2H2O media yielded a value of 1.013 +/- 0.026 ml/g for the partial specific volume of this lipid. We have measured spin lattice relaxation rates for the methyl and methylenemethyne protons of the hydrocarbon chains of diphytanoyl phosphatidylcholine in bilayer vesicles over a range of temperatures and at two NMR frequencies (100 and 220 MHz). The observed relaxation rates for the methylene protons in this system were approximately twice those previously reported for dipalmitoyl phosphatidylcholine at comparable temperatures and resonance frequencies, whereas the relaxation rates measured for the methyl protons were greater than those of the straight chain lipid by an order of magnitude. Measurement of the spin lattice relaxation rates of the hydrocarbon protons of the diphytanoyl phosphatidylcholine in a 10 mol% mixture of the branched-chain lipid in a deuterated host lipid, diperdeuteropalmitoyl phosphatidylcholine, showed a discontinuity in the temperature dependence of the proton NMR longitudinal relaxation rates of the branched-chain lipid in the region of the gel to liquid crystalline phase transition temperature of the deuterated dipalmitoyl phosphatidylcholine host lipid. This result may be taken as evidence of lateral phase separation of a liquid cyrstalline phase enriched in diphytanoyl phosphatidylcholine from a gel phase enriched in diperdeuteropalmitoyl phosphatidylcholine at temperatures below the phase transition temperature of deuterated host lipid. This conclusion is supported by the observation of an abrupt change in the hydrocarbon methylene linewidth (at 100 MHz) of 10 mol% diphytanoyl phosphatidylcholine in diperdeuteropalmitoyl phosphatidylcholine over the temperature range where lateral phase separation is taking place according to differential thermograms.

Lipid Bilayers↗
PubMed1979

Structure of cytochrome a3-Cua3 couple in cytochrome c oxidase as revealed by nitric oxide binding studies.

The addition of NO to oxidized cytochrome c oxidase (ferrocytochrome c:oxygen oxidoreductase, EC 1.9.3.1) causes the appearance of a high-spin heme electron paramagnetic resonance (EPR) signal due to cytochrome a3. This suggests that NO coordinates to Cu+2a3 and breaks the antiferromagnetic couple by forming a cytochrome a+33-Cu+2a3-NO complex. The intensity of the high-spin cytochrome a3 signal depends on the method of preparation of the enzyme and maximally accounts for 58% of one heme. The effect of N-3 on the cytochrome a+33-Cu+2a3-NO complex is to reduce cytochrome a3 to the ferrous state, and this is followed by formation of a new complex that exhibits EPR signals characteristic of a triplet species. On the basis of optical and EPR results, a NO bridge between cytochrome a+23 and Cu+2a3 is proposed--i.e., cytochrome a+23-NO-Cu+2a3. The half-field transition observed at g = 4.34 in the EPR spectrum of this triplet species exhibits resolved copper hyperfine splittings with [A+2] = 0.020 cm-1, indicating that the Cu+2a3 in the cytochrome a+23-NO-Cu+2a3 complex is similar to a type 2 copper site.

Animals↗
PubMed1979

Inhibition of cholesterol uptake by the arterial wall in the intact animal.

This report presents evidence that 7-ketocholesterol effectively inhibits cholesterol uptake by the arterial wall in intact rabbits. The 7-ketocholesterol was administered to the animals by infusing isologous blood pre-incubated with the sterol. Each animal was used as its own control by the use of 3H and 14C-labelled cholesterols as tracers to measure the arterial cholesterol uptake during two successive infusions. The mean cholesterol uptake by the carotid and femoral arteries in the presence of 7-ketocholesterol was 18+/-3 nmoles/g, whereas the average of cholesterol uptake by these arteries was 53+/-5 nmoles/g when the animals were infused with isologous blood containing no 7-ketocholesterol. The mean concentration of 7-ketocholesterol in the animals blood ranged from 34 to 87 nmoles/ml. In preliminary experiments in vitro, pre-incubation of plasma with particulate 7-ketocholesterol caused inhibition of cholesterol uptake by perfused pig coronary arteries, although the particulate fraction of 7-ketocholesterol was removed prior to perfusion. The mean cholesterol uptake by the arteries perfused with plasma containing 7-ketocholesterol (240+/-20 nmoles/ml) was 96+/-21 nmoles/g, whereas the mean cholesterol uptake of the paired controls was 324+/-86 nmoles/g. The 7-ketocholesterol was found to bind to the lipoproteins and with higher affinity to red cells.

Animals↗
PubMed1978

The effects of the thermal prephase transition and salts on the coagulation and flocculation of phosphatidylcholine bilayer vesicles.

Absorbance measurements of sonicated dipalmitoyl phosphatidylcholine vesicles reveal two aggregation processes: flocculation and coagulation. Flocculation is only observed for samples in monovalent cationic salt solutions or in salt-free suspensions. This process is abolished in the presence of di- or trivalent cations. It is also found to be strongly temperature dependent, occurring only below the thermal prephase transition of the lipid. Dispersal of the flocculates is rapid but they re-form at a rate dictated by the hysteresis in the prephase transition. In contrast, coagulation is slow. The extent of coagulation does not seem to be strongly dependent on the temperature, the nature of the electrolyte or its concentration. The relation of the coagulated state to vesicle-vesicle fusion is briefly discussed.

Mathematics↗