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M C Phillips

Publications and source records attributed to M C Phillips.

At least 127 records · Page 7Linked to original sources
PubMed1984

Kinetics of phosphatidylcholine and lysophosphatidylcholine exchange between unilamellar vesicles.

The rates of exchange of phosphatidylcholine and lysophosphatidylcholine from unilamellar donor vesicles to acceptor vesicles of similar composition were followed in a protein-free system to establish the relationship between the rate of exchange and the aqueous-phase solubility of the lipid. Further, the rate of exchange of dimyristoylphosphatidylcholine (DMPC) between vesicles was examined over a range of temperatures to determine the effect of the lipid phase transition on the rate of lipid exchange. Intervesicular exchange of DMPC is faster than transbilayer exchange; lipid molecules in the outer monolayer of the bilayer exchange with t1/2 = 2.0 h at 37 degrees C. A discontinuity is observed in Arrhenius plots of DMPC exchange; the activation energy over the temperature range 27-45 degrees C is 70 kJ mol-1. The t1/2 for DMPC exchange extrapolated to 24.5 degrees C (the phase transition temperature of the donor bilayer) is 6.5 h and from temperatures below 24 degrees C is 82.6 h. The differences in the thermodynamic parameters of activation for DMPC exchange above and below 24.5 degrees C are 25 kJ mol-1 for the activation enthalpy and 197 J mol-1 K-1 for the activation entropy. These differences are similar to the enthalpy and entropy changes associated with the gel to liquid-crystalline phase transition of DMPC. The rate of exchange of lysopalmitoyl-phosphatidylcholine (LPPC) was difficult to measure since LPPC transfers rapidly to the columns used for separating donor and acceptor vesicles; the t1/2 for transfer is less than 2 min. LPPC at 5 mol % in cholesterol-egg PC vesicles does not affect the rate of cholesterol exchange.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbon Radioisotopes↗
PubMed1984

Cholesterol transfer from small and large unilamellar vesicles.

The rates of transfer of [14C]cholesterol from small and large unilamellar cholesterol/egg yolk phosphatidylcholine vesicles to a common vesicle acceptor were compared at 37 degrees C. The rate of exchange of cholesterol between vesicles of identical cholesterol concentrations (20 mol%) did not differ from the rate of transfer from donor vesicles containing 20 mol% cholesterol to egg yolk PC vesicles. Further, the rate of transfer of [14C]cholesterol from vesicles containing 15 mol% dicetyl phosphate (to confer a negative charge) was not different from the rate of transfer from neutral vesicles. However, the half-time for transfer of [14C]cholesterol from large unilamellar donor vesicles was about 5-times greater (10.2 h, 80 nm diameter) than from small unilamellar vesicles (2.3 h, 23 nm diameter). These data suggest that increased curvature in small unilamellar vesicles reduces cholesterol-nearest neighbor interactions to allow a more rapid transfer of cholesterol into the aqueous phase.

Biological Transport, Active↗
PubMed1984

Influence of cholesterol on bilayers of ester- and ether-linked phospholipids. Permeability and 13C-nuclear magnetic resonance measurements.

13C-NMR and permeability studies are described for sonicated vesicles of phosphatidylcholines bearing two 16-carbon saturated hydrocarbon chains with (a) one ether linkage at carbon 1 (3) or 2 of glycerol and one ester linkage at carbon 2 or 1 (3) of glycerol; (b) two ether linkages and (c) two ester linkages at carbons 1 (3) and 2 of glycerol. The results of 13C-NMR relaxation enhancement measurements using cholesterol enriched with 13C at the 4 position indicate that no significant relocation of the cholesterol molecules takes place in the bilayer when a methylene group is substituted for a carbonyl group in phosphatidylcholine. The 4-13C atom of cholesterol undergoes similar fast anisotropic motions in diester- and diether -phosphatidylcholine bilayers, as judged by spin-lattice relaxation time measurements in the liquid-crystalline phase; although the fast motions are unaltered, linewidth and spin-spin relaxation time measurements suggested some restriction of the slow motions of cholesterol molecules in bilayers from phosphatidylcholines containing an O-alkyl linkage at the sn-2 position instead of an acyl linkage. At temperatures above the gel to liquid-crystal phase transition, the kinetics of ionophore A23187-mediated 45Ca2+ efflux from vesicles prepared from each type of phosphatidylcholine molecule were the same; the kinetics of spontaneous carboxyfluorescein diffusion from diester- and diether -phosphatidylcholine vesicles were the same, whereas mixed ether/ester phosphatidylcholine molecules gave bilayers which are less permeable. The rate constants were reduced on cholesterol incorporation into the bilayers of each type of phosphatidylcholine molecule. The reductions were not statistically significant for 45Ca2+ release. The rate constants for carboxyfluorescein release were also reduced by cholesterol to the same extent in vesicles from diester-, diether -, and 1-ether, and 1-ether-2-ester-phosphatidylcholines; however, a smaller reduction was noted in bilayers from the 1-ester-2-ether analog. The results provide further evidence that there are no highly specific requirements for ester or ether linkages in phosphatidylcholine for cholesterol to reduce bilayer permeability. This is a reflection of the fact that in both diester- and diether -phosphatidylcholine bilayers, the 4-13C atom of cholesterol is located in the region of the acyl carboxyl group or the glyceryl ether oxygen atom.

Cholesterol↗
PubMed1984

The conformation of apolipoprotein E isoforms in phospholipid complexes and their interaction with human Hep G2 cells.

The human hepatoma cell line, Hep G2, has been used to compare the metabolism by isolated liver cells of purified isoforms of human apolipoprotein E (apo E). Complexes of [125I]apo E-3/3, 2/2, 3/2 and 4/3 with dimyristoyl phosphatidylcholine (DMPC) were prepared by a detergent-dialysis method: discoidal, bilayer complexes with a stoichiometry of 125 +/- 15 mol DMPC/mol apo E resulted. The predominant phenotype apo E-3/3, and the phenotype apo E-2/2 characteristic of patients with Type III hyperlipoproteinemia, interact similarly with DMPC and adopt the same conformation with 60-70% alpha-helix, as monitored by circular dichroism spectroscopy. The uptake and degradation at 37 degrees C, and binding at 4 degrees C by Hep G2 cells, of [125I]apo E-3/3/DMPC and [125I]apo E-2/2/DMPC complexes were compared. Apo E-3/3 was degraded more rapidly than apo E-2/2 suggesting that the diminished catabolism of the latter phenotype by intact livers is due to lack of recognition by the hepatocytes. The observed degradation of apo E was 3-4 times greater than that which could be attributed to fluid phase endocytosis and low-affinity adsorptive endocytosis. The degradation of [125I]apo A-I by Hep G2 cells can be accounted for by the above endocytotic mechanisms. The distinction between apo E-3/3 and apo E-2/2 isoforms is attributed to the presence of a cell-surface receptor on Hep G2 cells which binds apo E-3/3 with a higher affinity than apo E-2/2.

Apolipoproteins↗
PubMed1984

Pericardial effusion: relation of clinical echocardiographic and electrocardiographic findings.

To evaluate the effects of pericardial effusion on the ECG, we compared clinical, echocardiographic and ECG findings in 459 patients. The prevalence of echocardiographic effusion ranged from 1% (1/79) among normal subjects, to 28% (32/114) among patients with valvular disease, 30% (27/90) in patients with hypertension, and 86% (18/21) in patients with pericardial disease. No relationship existed between left ventricular function and the prevalence of effusion, but a strong inverse relationship was found between LV function and effusion size (r = -0.63, p less than 0.01). Small and moderate sized effusions had a progressive damping effect on ECG voltage, displacing the regression lines between Sokolow -Lyon voltage and left ventricular mass downward by 1.2 and 4.4 mm respectively. Standard ECG criteria for low voltage (leads I, II, III each less than 0.5 mV, or V1 to V6 each less than 1.0 mV) were extremely insensitive for detection of effusions (12%), although highly specific (94%). Other ECG criteria which improved sensitivity resulted in an unacceptably high prevalence of false-positive diagnoses of pericardial effusion. Thus, echocardiographic effusions occur in only 1% of normal subjects but in more than 25% of patients with hemodynamic loading conditions, with a strong relationship between worsening left ventricular function and increasing effusion size. In contrast to the close relationship between echocardiographic pericardial effusions and clinical findings, low electrocardiographic QRS voltage is a weak predictor of the presence of pericardial effusion.

Adolescent↗
PubMed1983

The helical hydrophobic moments and surface activities of serum apolipoproteins.

The mean helical hydrophobic moments (muH) have been used to compare the amphipathic helices of several apolipoprotein classes with the helices in membrane proteins, water-soluble globular proteins and surface-active peptides. The amphipathic helices in serum apolipoproteins have similar muH and mean hydrophobicities to helices in water-soluble globular proteins. The intrinsic surface activities of proteins and peptides, as determined by surface pressure at the air/water interface, correlate with the product (muH . F) where muH is the average value of muH for all helices in the molecule, and F is the fraction of alpha-helix structure in the protein.

Apolipoproteins↗
PubMed1983

Cellular cholesteryl ester clearance. Relationship to the physical state of cholesteryl ester inclusions.

The hypothesis that clearance of cellular cholesteryl ester deposits may be a function of the physical state of the stored lipid has been investigated. Cultured rat hepatoma cells were induced to store cholesteryl ester in either anisotropic inclusions by exposure to free cholesterol-rich phospholipid dispersions or isotropic inclusions by exposure to identical dispersions supplemented with oleic acid. Differential scanning calorimetry demonstrated an order/disorder transition at 43 degrees C for cholesteryl esters stored in anisotropic inclusions; the enthalpy of this transition was consistent with a smectic liquid crystalline to liquid transition. Lipids in cells with isotropic inclusions displayed no order/disorder transitions over the range 20-80 degrees C, indicating that the lipids are in a liquid state. The presence of oleic acid did not influence the mass of cholesteryl ester stored but increased the amount of stored triglyceride. Fatty acyl compositions of the cholesteryl esters were different under the two loading conditions; in particular, there was 38% cholesteryl oleate in anisotropic inclusions and 65% cholesteryl oleate in isotropic inclusions. Kinetics of cholesteryl ester clearance from cells with either anisotropic or isotropic inclusions were studied during a 12-h exposure to acceptors of free cholesterol. In both cases, cholesteryl ester clearance is essentially linear over 12 h and is directly proportional to the initial content of cholesteryl ester. However, the fraction of initial content of cholesteryl ester cleared in 12 h is 0.17 +/- 0.05 for cells with anisotropic inclusions and 0.34 +/- 0.09 for cells with isotropic inclusions. Our data demonstrate that the more rapid clearance of cholesteryl ester by cells with isotropic inclusions can be correlated with the physical state of the cholesteryl ester.

Animals↗
PubMed1983

A comparison of the surface activities of rat plasma apolipoproteins C-II, C-III-0, C-III-3.

Rat apolipoproteins C-II, C-III-0 and C-III-3 give similar surface pressure (pi)-molecular area isotherms when spread at the air/water interface. When allowed to adsorb to the clean air/water interface, the intrinsic surface activity of apolipoprotein C-II is somewhat higher than that of apolipoprotein C-III. All three apolipoprotein C molecules can penetrate an egg phosphatidylcholine monolayer spread at the air/water interface causing an increase (delta pi) in surface pressure. Increasing the initial surface pressure (pi i) of the lipid monolayer decreases delta pi, and delta pi = 0 when pi i greater than or equal to 32 +/- 2 mN . m-1 for all three apolipoprotein C proteins. This implies that apolipoproteins C-II, C-III-0 and C-III-3 would adsorb and desorb similarly from the surfaces of lipoprotein particles during metabolism.

Adsorption↗
PubMed1983

Electrocardiographic recognition of left atrial enlargement.

The ECG is widely used as a screening test for left atrial enlargement (LAE). Surprisingly, the most widely used criterion of LAE, the P-terminal force in lead V1 (PTF-V1) has not been systematically evaluated to determine the optimal level of PTF-V1 for detection of LAE in clinical populations. Accordingly, we examined the relationship between PTF-V1 and left atrial size by echocardiogram in 361 patients and performed a Bayesian analysis of test performance in populations with a varying prevalence of LAE. As PTF-V1 increased from greater than or equal to 0.03 to greater than or equal to 0.08, sensitivity in the 82 patients with LAE (LA dimension greater than 40 mm) fell from 51% to 23%, and specificity rose from 70% to 93%. In our study population (LAE prevalence = 23%), diagnostic performance of criteria was: PTF-V1 greater than or equal to 0.03 greater than or equal to 0.04 greater than or equal to 0.05 greater than or equal to 0.06 greater than or equal to 0.08 Positive Predictive Accuracy 33 46 52 58 50 Negative Predictive Accuracy 83 83 84 83 80 Per Cent Correct Diagnosis 66 76 78 80 77 Positive predictive accuracy and per cent correct diagnosis improved progressively as PTF-V1 rose from greater than or equal to 0.03 to greater than or equal to 0.06, but fell at greater than or equal to 0.08. Applying our sensitivity and specificity data to Bayesian analysis, PTF-V1 greater than or equal to 0.06 performed best in all populations with prevalence of LAE less than or equal to 50%. We conclude that use of PTF-V1 greater than or equal to 0.06 is superior to the standard criterion of PTF-V1 greater than or equal to 0.04 for all purposes ranging from screening of a general population to evaluation of diseased individuals whose likelihood of LAE ranges up to 50%.

Adolescent↗
PubMed1983

Geometric determinants of electrocardiographic left ventricular hypertrophy.

Experimental studies have suggested that electrocardiographic recognition of left ventricular hypertrophy depends on geometric relationships involving wall thickness and chamber size. To determine the clinical significance of these observations, we studied the effects of echocardiographic LV mass (LVM), posterior wall thickness (PWT), interventricular septal thickness (IVST) and internal dimension (LVID) on ECG voltage in 360 patients. Standard voltage and nonvoltage manifestations of LVH correlated modestly with LVM (r = 0.33-0.44, p less than 0.001). Sokolow-Lyon precordial voltage (SLV) (SV1 + RV5 or V6) correlated moderately with LVM (r = 0.41, p less than 0.001), but correlated less well with IVST (r = 0.26), PWT (r = 0.24) or LVID (r = 0.22). Stepwise regression revealed that there was no relation, independent of LVM, between SLV and IVST (r = 0.03), PWT (r = 0.03) or LVID (r = 0.01). The 90 patients with increased LVM (greater than 215 g) but without LVH by SLV (false negatives) were compared with the 48 identified by SLV (true positives). False negatives differed from true positives in LVM (298 +/- 72 vs 339 +/- 98 g, p less than 0.01), age (55 +/- 18 vs 44 +/- 19 years, p less than 0.001), weight (70 +/- 16 vs 63 +/- 14 kg, p less than 0.02), and distance from skin to the interventricular septum (42 +/- 10 vs 38 +/- 8 mm, p less than 0.02). Thus, for a given LVM, ECG voltage criteria of LVH are independent of LV chamber dilatation or other geometric variables, but depend on age, weight and LV depth in the chest, suggesting that stratification of subjects by clinical variables has promise for improved electrocardiographic recognition of LVH.

Adolescent↗
PubMed1982

Kinetics and mechanism of free cholesterol exchange between human serum high- and low-density lipoproteins.

The mechanism of cholesterol and phosphatidylcholine (PC) exchange between human serum lipoproteins has been investigated by following the transfer of radiolabeled cholesterol and PC between high-density lipoprotein (HDL) and low-density lipoprotein (LDL). Initially, [14C]cholesterol was present in the donor lipoprotein particle which was either HDL2, HDL3, or LDL. After incubation in saline solution for various times, the HDL and LDL were separated by precipitation of the LDL with Mn2+-heparin reagent. More than 90% of the [14C]cholesterol in donor HDL3 is transferred to LDL in a first-order process whose half-time is 2.9 min at 37 degrees C. This indicates that transfer of cholesterol molecules from the cholesterol ester/triglyceride core of HDL to the phospholipid/apoprotein monolayer at the surface of the particle is not rate limiting for exchange. The half-time for dipalmitoyl-PC exchange from HDL3 to LDL is 5 +/- h, indicating that the flux of PC is much lower than that of cholesterol. The half-times for [14C]cholesterol exchange from HDL2 and LDL at 37 degrees C are 4 and 45 min, respectively. The interfacial fluxes at 37 degrees C from the various lipoproteins are 4, 15, and 10 cholesterol molecules/(10 nm2 h), respectively, for LDL, HDL2, and HDL3. The rate of labeled cholesterol transfer from HDL3 is not affected when the concentration of LDL acceptor is increased 40-fold. The activation energies of cholesterol transfer between 4 and 37 degrees C for HDL3, HDL2, and LDL are 70 +/- 3, 75 +/- 3, and 78 +/- 3 KJ/mol, respectively. The general characteristics of the process of exchange of cholesterol between lipoproteins resemble those for exchange between small unilamellar vesicles. The results are only consistent with a mechanism of exchange in which cholesterol molecules diffuse through the aqueous phase; the experimental activation energy is associated with desorption of lipid from the donor lipoprotein into the aqueous phase.

Cholesterol↗
PubMed1982

Mechanism of dissociation of human apolipoprotein A-I from complexes with dimyristoylphosphatidylcholine as studied by guanidine hydrochloride denaturation.

The reversibility of the binding of human apolipoprotein A-I (apo A-I) to phospholipid has been monitored through the influence of guanidine hydrochloride (Gdn-HCl) on the isothermal denaturation and renaturation of apo A-1/dimyristoylphosphatidylcholine (DMPC) complexes at 24 degree C. Denaturation was studied by incubating discoidal 1:100 and vesicular 1:500 mol/mol apo A-I/DMPC complexes with up to 7 M Gdn-HCl for up to 72 h. Unfolding of apo A-I molecules was observed from circular dichroism spectra while the distribution of protein between free and lipid-associated states was monitored by density gradient ultracentrifugation. The ability of apo A-I to combine with DMPC in the presence of Gdn-HCl at 24 degrees C was also investigated by similar procedures. In both the denaturation and renaturation of 1:100 and 1:500 complexes, the final values of the molar ellipticity and the ratio of free to bound apo A-I at various concentrations of Gdn-HCl are dependent on the initial state of the lipid and protein; apo A-I is more resistant to denaturation when Gdn-HCl is added to existing complexes than to a mixture of apo A-I and DMPC. There is an intermediate state in the denaturation pathway of apo A-I/DMPC complexes which is not present in the renaturation; the intermediate comprises partially unfold apo A-I molecules still associated with the complex by some of their apolar residues. Complete unfolding of the alpha helix and subsequent desorption of the apo A-I molecules from the lipid/water interface involve cooperative exposure of these apolar residues to the aqueous phase. The energy barrier associated with this desorption step makes the binding of apo A-I to DMPC a thermodynamically irreversible process. Consequently, binding constants of apo A-I and PC cannot be calculated simply from equilibrium thermodynamic treatments of the partitioning of protein between free and bound states. Apo A-I molecules do not exchange freely between the lipid-free and lipid-bound states, and extra work is required to drive protein molecules off the surface. The required increased in surface pressure can be achieved by a net mass transfer of protein to the surface; in vivo, increases in the surface pressure of lipoproteins by lipolysis can cause protein desorption.

Apolipoprotein A-I↗
PubMed1982

Lipid-protein interactions. Effect of apolipoprotein A-I on phosphatidylcholine polar group conformation as studied by proton nuclear magnetic resonance.

Spin-spin coupling constants derived from high-resolution 1H NMR spectra of pure 1-myristoyl-sn-glycero-3-phosphocholine (MLPC) micelles and 60:1 mol/mol been analyzed in order to determine the effects of apoprotein on phosphatidylcholine (PC) polar group conformation. The shift ratios of the polar group proton resonances after addition of the paramagnetic shift reagent Fe(CN)6(3-) to the above MLPC systems, egg PC small unilamellar vesicles, and human HDL3 have been used to compare the PC polar group conformations in all systems. The location of the largely alpha-helical apo A-I molecules in the complex with MLPC was deduced from its effects on the chemical shifts and spin-lattice relaxation times (T1) of the well-resolved 1H resonances from the various parts of the lipid molecules. The data are consistent with the apo A-I molecules lying in the surface fo the MLPC micelle with their amphipathic, alpha-helical segments intercalated among the glycerophosphocholine groups of the lipid molecules so that aromatic amino acid side chains are interspersed among the lipid hydrocarbon chains. This leads to a spacing out of the glycerol backbones and immediately adjacent methylene groups of the MLPC molecules, thereby causing an enhancement of the motions affecting T1. The presence of apo A-I at the lipid-water interface apparently does no perturb the PC polar group conformation, indicating that this conformation is determined by intramolecular effects. The preferred conformation of the phosphocholine group (Hauser, H., Pascher, I., Pearson, R. H., & Sundell, S. (1981) Biochim. Biophys, Acta 650, 21-51] is characterized by an almost exclusively gauche conformation of the choline group and predominantly antiperiplanar conformations about the C-C-O-P and P-O-C-C bonds. The PC molecules in MLPC micelles, MLPC-apo A-I complexes, egg PC vesicles, and HDL3 all have this polar group conformation.

Apolipoprotein A-I↗
PubMed1982

The stability and structure of cholesterol-rich codispersions of cholesterol and phosphatidylcholine.

In order to investigate the structure and stability of cholesterol-enriched dispersions of phosphatidylcholine (PC), cholesterol/dipalmitoyl phosphatidylcholine (DPPC) mixtures with molar ratios of 4 +/- 0.5/1 to 1/1 were dispersed in water by sonication. These dispersions comprise liposomes and unilamellar vesicles with diameters in the range 200-1800 A. The bilayers which have a repeat distance of 66 A in these particles at 20 degrees C can contain up to 4 mol cholesterol/mol PC when DPPC is used and about half this ratio with egg PC. these dispersions are metastable in that storage at either 4 or 20 degrees C leads to aggregation and precipitation of vesicles; in addition, there is a decrease in the cholesterol/PC molar ratio in the particles and formation of cholesterol monohydrate crystals. Cholesterol is released slowly and PC dispersions containing more than equimolar amounts of cholesterol can be stable for several months. The amount of free cholesterol was determined by differential scanning calorimetry from the heat associated with the transition at 157 degrees C from smectic liquid crystal to liquid cholesterol. Under conditions of gentle mixing, the maximum solubility of cholesterol in DPPC bilayers is 1.0 +/- 0.1 mol/mol PC when the mixture initially contains less than about 3 mol cholesterol/mol DPPC. This is consistent with published equilibrium phase diagrams which show that equimolar PC/cholesterol bilayers are stable in water.

Calorimetry, Differential Scanning↗
PubMed1981

Thermodynamic and molecular basis for dissimilar cholesterol-solubilizing capacities by micellar solutions of bile salts: cases of sodium chenodeoxycholate and sodium ursodeoxycholate and their glycine and taurine conjugates.

The bile salts chenodeoxycholate (CDC) and its 7 beta-hydroxy epimer ursodeoxycholate (UDC) are administered therapeutically (as acids) to dissolve cholesterol gallstones in man. Since their micellarr solutions and those of their physiological conjugates differ strikingly in their capacities to solubilize cholesterol, we studied the interfacial and micellar properties of the epimers by a number of complimentary physical--chemical methods and correlated these with their solubilizing capacities. The critical micella concentrations (cmc) estimated by surface tension, dye titration, and turbidimetry were similar (1-5 mM), varying slightly with the bile salt species, the method employed, NaCl concentration (0-1 M), and temperature (10-50 degrees C). The weight-average aggregation number (number of monomers per micelle, nw) at the cmc, derived from Debye plots of conventional light-scattering data and from the mean hydrodynamic radii of the micelles obtained by quasi-elastic light-scattering spectroscopy, revealed no appreciable differences between the UDC-CDC epimers or between their conjugates. From the mean hydrodynamic radii, the taurine conjugates were found to form larger micelles (nw = 15-17) than the glycine conjugates (nw = 13) which in turn were larger than the free species (n w = 5), respectively. Consistent with previous experimental deductions, free and conjugated CDC micelles grew slightly in size with increases in total lipid concentration, but UDC micelles did not. With solubilization of cholesterol monohydrate, the mean sizes of UDC (13.4 A) and of CDC (13 A) micelles in 10 g/dL solutions did not change appreciably, even as the cholesterol saturation limit was reached. At the air-5 M NaCl (pH 2) interface, the glycine conjugates formed more expanded monomolecular films than the free acid, and both UDC and its glycine conjugate collapsed at surface pressures that were 10-20 mN m-1 lower than the collapse pressures of monolayers of CDC and its glycine conjugate. Similarly, adsorbed monolayers of ionized UDC and its taurine conjugate lowered the surface tension of water approximately 5 mN m-1 less than equimolar concentrations of CDC and its taurine conjugate. By employing high-performance reversed-phase liquid chromatography (HPLC), we measured the relative hydrophilic--hydrophobic properties of the bile salts and found a close correlation between HPLC mobility and cholesterol-solubilizing cpacity. Assuming a single cholesterol binding site per micelle, we estimated from the nw values and bile salt/cholesterol saturation ratios that the magnitude of the cholesterol binding constant (K) was 5.7 X 10(6) L/mol for unconjugated CDC and 2.5 X 10(5) L/mol for unconjugated UDC at 30 degrees C. These results suggest that the differences in cholesterol-solubilizing capacities of CDC and UDC and their conjugates are due to subtle differences in micellar structure, resulting from the axial or equatorial orientation of the 7-hydroxyl function and the various conjugating groups...

Chenodeoxycholic Acid↗
PubMed1981

Mechanism of cholesterol and phosphatidylcholine exchange or transfer between unilamellar vesicles.

The mechanism of cholesterol and phosphatidylcholine exchange has been investigated by following the transfer of radiolabeled cholesterol and phosphatidylcholine from negatively charged, unilamellar cholesterol-egg yolk phosphatidylcholine donor vesicles to neutral acceptor vesicles of similar composition. Vesicles were incubated in the absence of protein and were stable to fusion over the course of the experiment. At intervals, donor and acceptor vesicles were separated by passage through a column of DEAE-Sepharose; less than 1% of the charged and 80-95% of the neutral vesicles were recovered in the eluate. Over 12 h at 37 degrees C, 90% of the donor vesicle [4-14C]cholesterol was transferred to the acceptor vesicles in a first-order process whose half-time was 2.3 +/- 0.3 h. This indicates that transfer of cholesterol molecules from the inner to outer monolayer of the vesicle bilayer is not rate limiting in exchange. In contrast to cholesterol exchange, the half-time for 1-palmitoyl-2-oleoyl[1-14C]phosphatidylcholine exchange was 48 +/- 5 h so that more than six molecules of cholesterol were transferred for each molecule of phosphatidylcholine. The interfacial flux of cholesterol from the donor bilayer is 5.3 x 10(-15) mol cm-2 s-1 (approximately 3 molecules/min for an average vesicle) and is similar to fluxes observed in other systems where phosphatidylcholine or cholesterol ester exchange is catalyzed by an exchange protein. When the acceptor vesicle concentration was increased 20-fold in cholesterol exchange experiments or 9-fold in phosphatidylcholine exchange experiments, the rate of label transfer was not affected. The activation energy of cholesterol exchange between 15 and 37 degrees C was 73 +/- 5 kJ mol-1. Transfer of cholesterol across a dialysis membrane is shown to be a slow process whose rate may be predicted by application of Fick's first law of diffusion. These results are only consistent with a mechanism of lipid exchange in which cholesterol and phosphatidylcholine diffuse through the aqueous phase; the experimental activation energy is associated with desorption of lipid from the donor bilayer into the aqueous phase.

Carbon Radioisotopes↗