Interaction of a bacterial adenosine triphosphatase with phospholipid bilayers

Properties of Ca2+- or Mg2+-dependent ATPase in rat heart sarcolemma

Rat heart sarcolemma was shown to hydrolyze ATP in the presence of Ca2+ or Mg2+; Ka values for Ca2+ and Mg2+ were in the range of 0.58-0.67 and 0.72-0.83 mM, whereas Vmax values were 33-38 and 21-28 mumol Pi/mg per hr, respectively. Both Ca2+ ATPase and Mg2+ ATPase showed low- and high-affinity sites for ATP; the Km value for the low-affinity sites for both enzyme activities was 300-325 microM, whereas Km values for high-affinity sites were 75-85 and 100-108 microM, respectively. The pattern of nucleotide hydrolysis in the presence of Ca2+ was found to be different from that with Mg2+. Although both high concentrations of ADP and Pi inhibited the enzyme activities, Mg2+ ATPase was more sensitive to ADP and less sensitive to Pi in comparison to Ca2+ ATPase. Storage of sarcolemma at about 0 degrees C showed a greater increase in ATP hydrolysis with Ca2+ than with Mg2+. The inhibitory effect of Mg2+ on Ca2+ ATPase, unlike that of Ni2+, Co2+, and Cu2+, was more than that on Mg2+ ATPase. Treatment of membranes with sodium dodecylsulfate or deoxycholate produced a greater reduction in Mg2+ ATPase than in Ca2+ ATPase. These results further support the view that Ca2+ ATPase and Mg2+ ATPase may be two separate enzymes in heart sarcolemma. It is suggested that Ca2+-dependent ATPase may be involved in opening calcium channels for the entry of calcium, whereas Mg2+ ATPase may serve as a Mg2+ pump mechanism for the efflux of magnesium from the cardiac cell.

Electrogenesis from an ATPase-ATP-sodium pseudo pump

The short circuit current and the open circuit voltage responses of membranes to ATP, which have been attributed to membrane ATPase acting as a sodium pump, have been reproduced not only in a lipid membrane containing solubilized ATPase but also in membranes formed of the phospholipids contained in ATPase. The response is greatest with cardiolipin, but occurs with other acidic phospholipids. This observation of electrogenesis without hydrolysis is a surface phenomenon probably due to the alignment of ATP on the phospholipid by ion association at its interface with the water phase. The finding constitutes a precaution for interpreting studies of membrane Na-K-ATPase or for its incorporation into an artificial membrane. The substances necessary for electrogenesis are present at the mitochondrial membrane, and the particular orientation of the ATP on the phospholipids in vitro suggests a role for this ion association in the function of Na-K-ATPase.

Chloride flux in bilayer membranes: the electrically silent chloride flux in semispherical bilayers

High resistance semispherical bilayer membranes of areas as large as 0.3 cm-2 were formed from a decane solution of synthetic diphytanoylphosphatidylcholine. These bilayers had a specific resistance of about 10-9 omega cm-2 and a specific capacitance of 0.38 mu F cm- minus 2 at 20 degrees in 0.1 M KCL. Under these conditions, chloride permeability was 6.8 times 10- minus 8 cm/sec. This electrically silen 36-Cl flux was found to be about 10-3-fold larger than the chloride current calculated from the electrical parameters of the system. The chloride flux in the bilayer was independent of the applied electrical field and was unaltered by addition of reducing agents to the ambient aqueous solutions. It was, however, substantially reduced when NO3 minus was substituted for Cl minus on the side of the bilayer initially free of 36-Cl, or if I minus was added to the aquesous phases in the concentration range of 0.001-0.1 M. These results strongly suggested that the electrically silent flux of 36-Cl is primarily a carrier mediated diffusion process in which phosphatidylcholine acts as the carrier species.

NA + -selective ionophoric material derived from electric organ and kidney membranes

Material that increases black lipid-membrane (oxidized cholesterol) conductance has been demonstrated in the acid-soluble fraction of tryptic digests of membrane fractions from Electrophorus electric organ and beef kidney. The conductance change elicited by this material is highly selective for Na(+). The activity of the material was greatly enhanced by passage through DEAE-cellulose. Activity could be destroyed by further incubation with Pronase. Since conductivity increases exponentially with dose of ionophore, the conductive unit may be an oligomer.

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.

Formation of bimolecular membranes from lipid monolayers and a study of their electrical properties

Bimolecular membranes are formed from two lipid monolayers at an air-water interface by the apposition of their hydrocarbon chains when an aperture in a Teflon partition separating two aqueous phases is lowered through the interface. Formation of the membrane is monitored by an increase of the electrical capacity, as measured with a voltage clamp. Electrical resistance of the unmodified membrane is analogous to that of conventional planar bilayers (black lipid membranes) prepared in the presence of a hydrocarbon solvent, i.e., 10(6)-10(8) ohm cm(2); the resistance can be lowered to values of 10(3) ohm cm(2) by gramicidin, an antibiotic that modifies the conductance only when the membranes are of biomolecular thickness. In contrast to the resistance, there is a significant difference between the capacity of bilayers made from mono-layers and that of hydrocarbon-containing bilayers made by phase transition; the average values are 0.9 and 0.45 muF cm(-2), respectively. The value of 0.9 muF cm(-2) approximates that of biological membranes. Assuming a dielectric constant of 2.1 for the hydrocarbon region, the dielectric thickness, as calculated from a capacity of 0.9 muF cm(-2), is 22 A. This value is 6-10 A smaller than the actual thickness of the hydrocarbon region of bilayers and cell membranes, as determined by x-ray diffraction. The difference may be due to a limited penetration of water into the hydrocarbon region near the ester groups that would lower the electrical resistance of this region and reduce the dielectric thickness. Asymmetric membranes have been formed by adjoining two lipid monolayers of different chemical composition.

Microbial assimilation of hydrocarbons: phospholipid metabolism

An analysis of the turnover of the major phospholipids of Micrococcus cerificans growing or nongrowing cultures. The turnover rates of (14)C-PE and (14)C-PE were 61.5% of the total phospholipid, exhibited no significant rate of turnover in either growing or nongrowing cultures. The turnover rates of PE-(14)C and PE-(32)P were 3.2% per hr and 1.2% per hr, respectively. Phosphatidylglycerol (PG) exhibited a turnover rate of 11% and 7.7% per hr for (14)C and (32)P, respectively, indicating an extremely slow metabolism. PG metabolism was examined in greater detail, and the data indicated a preferential 75% incorporation of glycerol-1,3-(14)C into the unacylated portion of the PG molecule. The turnover of cardiolipin (CL) was extremely slow in growing cells whereas nongrowing cells exhibited a 30% and 36% increase per hr for (14)C-Cl and (14)C-CL, respectively. Glycerol-1,3-(14)C was not converted to phospholipid fatty acid carbon; all radioactivity appeared only in the water-soluble backbone of the phospholipids. The kinetics of assimilation of hexadecane-1-(14)C into cellular lipids is presented. Radioactivity in neutral lipid increased approximately sevenfold over the growth cycle, whereas radioactivity in phospholipid increased 50-fold during the same time period. The incorporation of radioactive fatty acids derived from the direct oxidation of hexadecane-1-(14)C demonstrated differential kinetics of assimilation into PE, PG, and CL. The results indicated a rapid turnover of phospholipid fatty acids in M. cerificans growing at the expense of hexadecane.

Fluorescence spectroscopy of an oriented model membrane

We have devised a simple method that makes it feasible to apply fluorescence techniques to lipid bilayer membranes to elucidate aspects of their structure and dynamics. Fluorescence excitation, emission, and polarization spectra were obtained from a single spherical bilayer membrane consisting of oxidized cholesterol and fluorescent probe. The emission transition moments of N,N'-di(octadecyl)oxacarbocyanine and 12-(9-anthroyl)-stearic acid were found to be aligned parallel to the plane of the bilayer, whereas that of p-bis-[2-(4-methyl-5-phenyloxazolyl)]-benzene was aligned in a perpendicular direction. All three probes exhibited appreciable rotational mobility, parallel to the plane of the bilayer, in durations of nanoseconds. An attractive feature of this model membrane is that fluorescence measurements can be made at the same time as electrical measurements and perturbations. Also, it may be possible to incorporate functional protein assemblies into this model and to use fluorescence spectroscopy to delineate some aspects of their assembly and function.