Phosphatidylcholesterol bilayers. A model for phospholipid-cholesterol interaction

Aqueous dispersions of monovalent and divalent cation salts of O-(1,2-dipalmitoyl-sn-glycero-3-phosphoryl) cholesterol form multilamellar vesicles as shown by freeze-fracture electron microscopy, by electron micrographs of the negatively stained liposomes, and by swelling curves of liposomes in hypoosmotic medium. Differential scanning calorimetry reveals that aqueous dispersions of divalent metal salts of O-(1,2-dipalmitoyl-sn-glycero-3-phosphoryl)-cholesterol undergo a characteristic thermotropic phase transition with a relatively large cooperative unit (n > 250 for the calcium salt). In contrast, monovalent cation salts of O-(1,2-dipalmitoyl-sn-glycerol-3-phosphoryl)cholesterol do not show a thermotropic phase transition under comparable conditions. The molecular area of O-(1,2-dipalmitoyl-sn-glycero-3-phosphoryl)cholesterol in a monolayer is the same in the presence and absence of Ca²⁺, and is virtually equal to the area of an equimolar mixture of dipalmitoyl phosphatidic acid and cholesterol. To account for the novel state induced by Ca²⁺ on aqueous dispersions of O-(1,2-dipalmitoyl-sn-glycero-3-phosphoryl)cholesterol (i.e., bilayer organization and highly cooperative phase transition), a linear array model is proposed in which Ca²⁺ bridges adjacent arrays of O-(1,2-dipalmitoyl-sn-glycero-3-phosphoryl)cholesterol molecules, thus freezing the acyl chains in their normal state. One of the main corollaries of the model is that the cooperative unit for a thermotropic phase transition is essentially one-dimensional, rather than a two-dimensional matrix. O-(1,2-Dipalmitoyl-sn-glycero-3-phosphoryl)cholesterol is proposed as an orientationally and conformationally restricted analog of glycerophospholipid plus cholesterol in bilayers.     

Pressure-induced changes in the molecular organization of a lipid-peptide complex. Polymyxin binding to phosphatidic acid membranes

The effect of 100 atm pressure on the organization of the lipid-peptide complex formed between polymyxin and dipalmitoyl phosphatidic acid has been investigated. Phase transition curves were obtained by electron paramagnetic resonance by measuring the partition coefficient of the spin label, 2, 2, 5, $\text{5-}\text{tetramethylpiperidine}\text{-N-}\text{oxyl}$. The three-step phase transition curve previously obtained with fluorescence polarization measurements was confirmed, demonstrating three distinct phosphatidic acid domains in the bilayer. Pressure increases binding of polymyxin to phosphatidic acid bilayers and alters the proportions of the two domains that differ in the mode of binding between phosphatidic acid and polymyxin. The binding curves of polymyxin to phosphatidic acid bilayers were determined and it was shown that application of pressure reduces the cooperativity of the binding curve.     

Dynamic stiffness and crossbridge action in muscle

Small sinusoidal vibrations at 300 Hz were applied to frog sartorius muscle to measure the dynamic stiffness (Young's modulus) throughout the course of tetanus. For a peak-to-peak amplitude of 0.4% the dynamic Young's modulus increased from 1.5×10⁵ Nm^–2 in the resting state to 2×10⁷ Nm^–2 in tetanus. After correction for the external connective tissue, the dynamic Young's modulus of the muscle was almost directly proportional to the tension throughout the development of tetanus. The ratio of dynamic Young's modulus to tensile stress thus remained constant (with a value at 300 Hz of approximately 100), consistently with Huxley and Simmons' identification of the crossbridges as the source of both tension and stiffness.For a single crossbridge the ratio of stiffness to tension was 8.2×10⁷ m^–1 at 300 Hz; it is deduced from literature data that the limiting value at high frequencies is about 1.6×10⁸ m^–1. This ratio is interpreted on Harrington's (1971) model to show that crossbridge action can be explained by a helix-coil transition of about 80 out of the 260 residues in each S-2 myosin strand. It is also shown that a helix-coil model can account for the observed rapid relaxation of muscle without invoking any complex behaviour of the crossbridge head.     

Differential scanning calorimetry of milk fat globule membranes

Differential scanning calorimetry was employed as an aid in examining the structure of the bovine milk fat globule membrane. At least six major endotherms are observed between 10 and 90°C, corresponding to order-disorder transitions of discrete structural domains of the membrane. These endothermic transitions occur at 16, 28, 43, 58, 68, and 75°C. The transitions occurring between 10 and 50°C were reversible, suggesting the involvement of lipid. However, the high temperature transitions were irreversible. The calorimetric C transition, centered at 43°C, was shown to involve neutral lipid, since the endotherm was reversible, insensitive to proteolysis, and similar to the endotherm of the isolated neutral lipid fraction of the milk fat globule membrane. The glycolipid and phospholipid fractions of the milk fat globule membrane yielded endotherms outside of the temperature range of the C transition. Another endotherm, the D transition (58°C), was found to involve the denaturation of the major membrane coat protein, butyrophilin (band 12). Evidence for this assignment included the following observations: (i) the nearly selective proteolysis of butyrophilin resulted in the complete removal of the D transition, (ii) the butyrophilin-enriched, Triton X-100-insoluble pellet of milk fat globule membrane yielded a relatively normal D transition, and (iii) the irreversible, disulfide-stabilized aggregation of butyrophilin occurred in the membrane solely at the temperature of the D transition. Furthermore, no other prominent milk fat globule membrane polypeptide formed these non-native disulfide crossbridges during the D transition. The sources of the other major endotherms of the milk fat globule membrane have not yet been assigned.     

Protein-lipid interactions and differential scanning calorimetric studies of bacteriorhodopsin reconstituted lipid-water systems

Bacteriorhodopsin has been reconstituted at various molar concentrations into liposomes of dimyristoyl- and also of dipalmitoylphosphatidylcholine. Differential scanning calorimetry indicates that as the protein concentration within the lipid bilayer increases, the cooperativity of the lipid phase transition is reduced, i.e. the transition is broadened, while the midpoint transition temperature remains virtually unchanged. Freeze-fracture electron microscopy of our preparation shows, in agreement with previous data from other laboratories, that extensive protein aggregation occurs when the liposome is cooled below the T_c transition temperature of the lipid. Laser flash photolysis measurements of protein rotation of the bacteriorhodopsin show, especially in the case of protein-rich recombinants, that protein aggregates exist even above T_c. The perturbation caused by the presence of bacteriorhodopsin in the lipid bilayer is similar to that produced by other intrinsic proteins. The difficulty of correlating the observed calorimetric enthalpy data with a simple concept of a ‘boundary lipid layer’ based upon consideration of a single isolated protein is discussed in view of the occurrence of protein aggregates both above and below T_c. It is concluded that the reduction of enthalpy is related to the number of lipids which solvate the protein aggregates within the protein-lipid patches and are thereby removed from the cooperative melting and enthalpy of the remaining regions of pure lipid.     

Protein-catalyzed phospholipid exchange in bilayer vesicles determined by flow cytometry and electron microscopy

The protein-induced lipid transfer between phosphatidylcholine vesicles was investigated. Measurements of the degree of polarization at single vesicles were made by flow cytometry using diphenylhexatriene as the optical probe. Vesicles differing in phase transition temperature could be distinguished by their degree of polarization at a temperature where one population was in the fluid ($\text{T > T}{}_{\mathrm{<mtext>t</mtext>}}$) and the other one in the quasi-crystalline ($\text{T < T}{}_{\mathrm{<mtext>t</mtext>}}$) state. Besides vesicles containing exchanged lipids we also observed fractions of unaffected vesicles. The lipid exchange was visualized directly by freeze-fracture electron microscopy. The characteristic ‘ripple’ structure of phosphatidylcholine vesicles disappeared upon exchange with lipid in the fluid state.     

Comparative study on the properties of saturated phosphatidylethanolamine and phosphatidylcholine bilayers: Barrier characteristics and susceptibility to phospholipase A2 degradation

Comparative studies on bilayer systems of saturated phosphatidylcholines and phosphatidylethanolamines revealed a maximum in ionic permeability in phosphatidylcholine bilayers at the temperature of the gel to liquid-crystalline phase transition but such an increase in permeability was not detectable in bilayers of phosphatidylethanolamine. Furthermore, it was found that at the phase transition temperature the phosphatidylcholine bilayers are subject to rapid hydrolysis by pancreatic phospholipase A₂ whereas phosphatidylethanolamine bilayers are not. These differences are discussed in view of detailed information on the molecular organization in the gel and liquid crystalline phases of the two phospholipid classes.     

Temperature dependence of the transition enthalpy of core particle DNA and of long DNA

The denaturation of short (145 base pairs) and long (about 8000 base pairs) DNA moelucules has been studied by adiabitic differential microcalorimetry in solutions with different NaCl content. It is found that the enthalpy of denaturation of short DNA is more sensitive to changes in T_m than that of long DNA. A comparison with other data is also given.     

Cell Cycling of Astrocytes and Their Precursors in Primary Cultures: A Mevalonate Requirement Identified in Late G1, but Before the G1/S Transition, Involves Polypeptides

The relationship between mevalonate and cell cycling was investigated in developing glial cells. Primary cultures of newborn rat brains were serum-depleted (0.1%, vol/vol) for 48 h on days 4-6 in vitro, then returned to 10% calf serum (time 0). After 48 h, 70-80% of the cells were glial fibrillary acidic protein (GFAP)-negative by indirect immunofluorescence; 79 +/- 7% were GFAP-positive after an additional 3 days. Serum shift-up resulted in 12 h of quiescence, and then by 20 h (S phase) in increased proportions of cells synthesizing DNA (from 15 +/- 6% to 75 +/- 4% by bromodeoxyuridine immunofluorescence at 12 h and 20 h, respectively) and rates of DNA synthesis (42 +/- 6 versus 380 +/- 32 cpm/micrograms of protein/h of [3H]thymidine uptake). Additional mevalonate (25 mM) for 30 min at 10 h reversed the inhibition of DNA synthesis apparent with mevinolin (150 microM), an inhibitor of mevalonate synthesis, present from time 0. Cycloheximide added simultaneously with mevalonate prevented this reversal of inhibition. To cause arrest at G1/S, cultures were exposed to hydroxyurea between 10 and 22 h. By 3 h after hydroxyurea removal, bromodeoxyuridine-labeled nuclei increased from 0% to 75 +/- 9%, and DNA synthesis increased 10-fold. Mevinolin failed to inhibit these increases. Thus, primary astroglial precursors stimulated to progress through the cell cycle express a mevalonate requirement in late G1, but before the G1/S transition. The effect of mevalonate was characterized further as being brief (30 min) and as requiring polypeptides.     

The Effect of Free Radicals Induced by Paraquat and Copper on the In Vitro Development of Plasmodium falciparum

The role of transition metals in paraquat toxicity was studied in cultures of Plasmodium falciparum. We showed that addition of copper led to an enhancement of the plasmodium killing, whereas addition of chelating agents. such as desferrioxamine and diethylenetriamine pentaacetic acid markedly reduced the toxic effects. Parsitized G6PD deficient erythrocytes were more sensitive than parasitized normal eryth-rocytes to copper and to the combination of copper and paraquat.