Studies on phosphatidylcholine vesicles. Formation and physical characteristics

Lipid-polypeptide interactions in bilayer lipid membranes

The modifications of the electrical properties of bilayer lipid membranes (BLM) composed of cholesterol and an ionic surfactant upon interaction with charged polypeptides were studied. The addition of 10(-8) M polylysine (Ps(+)) to one side of anionic cholesterol dodecylphosphate BLM increases the specific membrane conductance over 1000-fold (from 10(-8) to 10(-5) mho/cm(2)) and develops a cationic transmembrane potential larger than 50 mV. This potential is reverted by addition of polyanions such as RNA, polyglutamic or polyadenilic acid to the same side on which Ps(+) is present, by addition of Ps(+) to the opposite side, or by addition of trypsin to either side. Both conductance and potential changes are hindered by increasing the ionic strength or by raising the pH of the bathing medium, disappearing above pH 11.5 where it is known that Ps(+) folds into an α-helix. The interaction of polyglutamic acid (PGA) with a cationic cholesterol-hexadecyltrimethylammonium bromide BLM results in increased membrane conductance and development of an anionic transmembrane potential which is reverted by addition of polycations to the same aqueous phase where PGA is present. Addition of either Ps(+) or PGA to one or both sides of a neutral BLM composed of 7-dehydrocholesterol induces no significant change. The observations suggest the formation of a lipid polymer membrane resultant from the interaction, predominantly electrostatic, of the isolated components. The implications of these results are discussed in terms of the current models of membrane structure.

Measurement of transmembrane potentials in phospholipid vesicles

Phosphatidylcholine vesicles are permeable to tempotartrate, a spin-label derivative of tartaric acid. The inside-outside distribution of tempotartrate is coupled to the inside-outside distribution of H(+), so it must be a measure of the transmembrane electrical potential difference in vesicles permeable to H(+). This prediction is borne out by the finding that the inside-outside distribution of tempotartrate is the reciprocal of the inside-outside distribution of K(+) in vesicles prepared in the presence of valinomycin. The inside-outside distribution of tempotartrate is, by contrast, equal to the inside-outside distribution of Cl(-) in vesicles without valinomycin. This is evidence that an inside-outside Cl(-) concentration gradient induces an H(+) gradient, which must be due to HCl permeation.

Magnetic resonance studies on membrane and model membrane system: proton magnetic relaxation rates in sonicated lecithin dispersions

We have used Fourier transform nuclear magnetic resonance methods to measure the spin-lattice and transverse relaxation times at 220 MHz of the choline N-methyl and the fatty-acid alpha-carbonyl, allyl, vinyl, methylene, and methyl protons of sonicated egg-yolk lecithin. Over the temperature range investigated the T(1) values were, in general, similar to, but different from, one another, suggesting that the relaxation rates of all of the fatty-acid protons are not determined solely by spin-diffusion to a heat sink. Arrhenius plots of the T(1) data gave activation energies similar to those for the barriers to internal rotation in alkanes. The values of the transverse relaxation rate, T(2), showed a relatively large variation among the proton resonances; about 20% of the methylene protons had a T(2) of 56 msec, while the remaining protons relaxed according to a distribution of values all shorter than 20 msec. Such a distribution of relaxation times is envisioned to arise from a distribution of correlation times stemming from complex motions in which extended angular excursions of the fatty acid chain are coupled to trans-->gauche conformational transitions.

Lateral diffusion of phospholipids in a vesicle membrane

Nuclear-resonance spectra of phosphatidylcholine (PC) vesicles are broadened by spin-labeled PC. The broadening of the N-methyl proton line is proportional to spin-labeled PC concentration; the broadening by 1 molecule in 100 of spin-labeled PC is 9.7 Hz at 35 degrees C; the broadening is not due to collisions between vesicles, exchange of spin label between vesicles, or fusion of vesicles. These findings are proof of rapid diffusion in the plane of the PC bilayer (lateral diffusion). A further study of the broadening reveals the molecular frequency of the translation step for lateral diffusion to be much greater than 3 x 10(3) sec(-1) at 0 degrees C.

Fluorescent probes of biological membranes

FLUORESCENT AND PHOSPHORESCENT PROBES THAT HAVE READILY INTERPRETABLE EMISSION PROPERTIES CAN BE SPECIFICALLY INSERTED INTO BIOLOGICAL MACROMOLECULES TO REVEAL FACETS OF THEIR STRUCTURE AND DYNAMICS: (1) Proximity. Singlet-singlet and triplet-singlet energy transfer can serve as spectroscopic rulers in the 10-65 A range, whereas triplet-triplet transfer can be used to show that two groups are less than about 12 A apart. (2) Rotational mobility. Nanosecond fluorescence polarization measurements can reveal whether a macromolecular system has any modes of flexibility in times of nanoseconds. (3) Polarity. The presence of mobile dipoles in the environment of certain chromophores is reflected in their fluorescence quantum yield and emission spectrum. We have synthesized a number of new fluorescent probes for biological membranes. Anthroyl stearic acid (I), dansyl phosphatidyl ethanolamine (II), and octadecyl naphthylamine sulfonic acid (III) are readily incorporated into bilayer vesicles composed of phosphatidyl choline. The emission spectra of these probes in the vesicles indicate that the chromophore of I is located in the hydrocarbon region, that of II is located in the glycerol layer, and that of III is located at the aqueous interface of the bilayer. Thus, fluorescent chromophores can be selectively placed in different transverse regions of a model membrane system.

Interaction of a bacterial adenosine triphosphatase with phospholipid bilayers

A solubilized nonparticulate adenosine triphosphatase from Streptococcus fecalis spheroplast membranes (Abrams, A., and C. Baron, Biochemistry, 7,501 (1968)) interacts at pH 7.5 with lipid bilayer membranes to produce a 10(2)- to 10(4)-fold increase in the electric conductance of the bilayer. In addition, a small decrease in the electrical capacitance of the interactant system is also observed. Interaction is obtained with bilayers formed from a solution in n-decane of either purified egg phosphatidyl-choline, synthetic diphytanoyl phosphatidyl-choline, or a total extract of the membrane lipid of S. fecalis. The magnitude of the increase in conductance is dependent on the presence of Mg(++) and upon the concentrations of both Na(+) and K(+) in the range of 10(-2) to 10(-1)M. An additional tenfold increase in conductance is obtained when ATP is added to the ambient aqueous phase surrounding the bilayer ATPase interactant system. No conductance change is obtained with ATPase which has been treated with pronase. ATPase activity is dependent upon Mg(++) and upon the concentrations of Na(+) and K(+) in the region of 10(-1)M. The function of the ATPase in the intact bacterial membrane is apparently associated with the active transport of cations. The increased conductance in the bilayer which results from its interaction with the ATPase, together with the similarity between the dependence of this interaction and the dependence of ATPase activity on Mg(++), ATP, and the Na(+), K(+) concentrations suggests that the bilayer-ATPase interactant complex may be similar in structure and properties to the membrane-ATPase complex in the intact organism.