Asher Ilani

Dependence of photosensitivity of bileaflet lipid membranes upon the chlorophyll and carotenoid content.

SummaryBileaflet lipid membranes were formed from solutions containing lecithin, chlorophyll and carotene in various concentrations. If all the above components were present at sufficient concentrations the membranes were photosensitive; i.e., a photocurrent was produced if a redox potential gradient was present across the membranes. The presence of chlorophyll and carotene were essential for the photosensitivity of the membranes. Photoresponse could be elicited by illuminating the membrane with light which did not excite carotene. On the other hand, elimination of the part of the light spectrum which excites chlorophyll led to the abolition of the photoresponse. The findings of this study are consistent with the assumption that the excited chlorophyll chromophores allow electron exchange at the membrane-water interface while the presence of carotene allows electron movement across the “bulk” lipid membrane.

The pH dependence of the hemolytic potency of bile salts

The membrane damaging potential of dilute solutions of bile salts was evaluated by monitoring continuously the hemolysis of a small sample of red blood cells (RBC) introduced into a defined media containing the bile salts at various pH values. The strength of the hemolytic bile salt was characterized by the rate of the induced hemolysis and by the time that elapsed between the introduction of the RBC sample into the bile salt containing solution and the onset of hemolysis. The potency of the unconjugated bile acids was extremely sensitive to pH, e.g. the rate of hemolysis caused by a 7.5 mM cholate was 1.5%, 20% and 64% per min when the pH of the solution was 7.65, 7.3 and 6.85, respectively. At low pH values the membrane damaging effects of deoxycholate was clearly discerned at micromolar concentration range. The hemolytic potency of glycodeoxycholate was also enhanced significantly by lowering the pH. The taurine-conjugated cholate and deoxycholate were only slightly sensitive to variations in pH. Taurocholate at concentrations that were not hemolytic greatly enhanced the injurious potency of deoxycholate. These results imply that in acidic solutions the presence of bile acids can cause damage to cell membranes. It is suggested that the acidic environment in the proximal duodenum and acidosis developed during hypoxia in the liver are two situations in which the bile salts may constitute a pathogenic factor.

Bretylium opens mucosal amiloride-sensitive sodium channels

Addition of the quanternary ammonium compound, bretylium, to the outer surface of a frog skin leads to an increase in the potential difference and in the short circuit current across the skin. Bretylium does not have any effect when applied to the inside face of the frog skin. The effect of bretylium is dependent upon the presence of sodium ions in the outer medium; it is depressed when sodium is replaced by choline or potassium but not when lithium substitutes for sodium. The bretylium effect is blocked by the specific sodium channel blocker, amiloride. It is proposed that bretylium opens mucosal, amiloride-sensitive sodium channels.

PHOTOINDUCED ELECTRON TRANSFER ACROSS LIPID BILAYERS CONTAINING MAGNESIUM OCTAETHYLPORPHYRIN

Abstract

The effect of prymnesin on the electric conductivity of thin lipid membranes

SummaryA proteolipidic toxin, prymnesin, when added to the aqueous solutions around thin lipid membranes causes a marked increase in membrane conductance. The toxin-treated membrane is cation-permselective. The extent of cation permselectivity is dependent upon ionic strength of the aqueous solutions in a fashion similar to the dependence of cation permselectivity of a cation exchanger containing about 100mm of fixed negative sites. Dose-response relationship studies reveal a linear relation between log prymnesin concentration and log membrane conductance. The slope of the curve is around 3 if the toxin is applied to one side of the membrane and is around 7 if the toxin is applied to both sides of the membrane. The membrane treated with toxin on one side only is clearly asymmetric in its properties. These characteristics are expressed by an asymmetric current-voltage relationship, and by asymmetric sensitivity of membrane conductance to pH and to salt concentration. The conductance of the toxin-treated membrane is inversely proportional to temperature. It is suggested that aggregates of toxin moieties assemble in the membrane to form negatively charged aqueous pores. There is roughly a good correlation between the increase in membrane conductance and the increase in membrane permeability to urea if both were attributed to the formation of aqueous channels in the membrane.

Hydrogen ions in hydrophobic membranes

Filters containing fixed negative charges were saturated with hydrophobic solvent and exposed to solutions containing K+, Na+, or tetraethylammonium+ at various pHs. Between pH 2 and pH 7, H+ is comparable to K+ and tetraethyl-ammonium+ in its ability to compete for fixed negative sites in the membrane. The membranes, however, show much greater selectivity for H+ than for Na+. Mobility of H+ in the membrane is dependent upon the nature of the counterion in the membrane. However, in all conditions examined, its mobility is much lower than that of K+ or Na+ and is approx. one-third that of tetraethylammonium+. The bi-ionic potential of H+vs. tetraethylammonium+ is around 70 mV, and it tallies with the observed relative mobility and selectivity for these ions. Corroborating previous findings, these facts support the view that ion movement in such membranes does not occur via continuous water channels but involves passage of the ions through the hydrophobic medium.

Comparison between bretylium and diphenylhydantoin interaction with mucosal sodium-channels

The antifibrillatory drug bretylium and the antiepileptic drug diphenylhydantoin cause an increase in the potential different and in the short-circuit current (SCC) across frog skin when added to the outer surface. The effect of both drugs depends upon the presence of sodium ions in the outer medium and is blocked by the specific sodium channel blocker, amiloride. Quantitative analysis shows that amiloride binds to open as well as closed mucosal sodium channel with the same affinity. The effects of diphenylhydantoin and bretylium differ with respect to their dependence on external pH. The diphenylhydantoin or the bretylium stimulatory effects are additive to the effects of oxytocin. In most cases the diphenylhydantoin and bretylium effects are also additive. It is concluded that the external side of the mucosal Na+ channels contains sites which interact specifically with either bretylium or diphenylhydantoin and thus remove the sodium induced closure of the channels.

SODIUM-DEPENDENT TRANSPORT OF PHOSPHATE IN NEURONAL AND RELATED CELLS

Sodium-dependent phosphate entry into neuronal cells was demonstrated in synaptic plasma membrane vesicles and synaptosomes prepared from rat brains, in PC12 cells and in primary culture of pituitary cells. The extent of the sodium-dependent phosphate transport in the synaptic plasma membrane preparation, at [Na]out = 110 mM and [P(i)]out = 0.1 mM, varied between 0.28 to 1.02 nmol phosphate/mg membrane protein/min. In pituitary cells the value was only about 0.05 nmol P(i)/mg protein/min. In PC12 cells the activity increased from 0.0085 to 0.26 nmol P(i)/mg protein/min in the transit from undifferentiated to differentiated cells. The dependence of phosphate on sodium concentrations fits a model in which two sodium ions are required to transfer the phosphate into the cells with a K[Na]0.5 of 43 mM. The K(m) for the phosphate transport in the synaptic plasma membrane preparations was between 0.1 and 0.45 mM. It is concluded that sodium-driven active transport of phosphate is a ubiquitous activity in various types of neuronal cells.

Discrimination between monovalent and divalent cations by hydrophobic solvent-saturated membranes containing fixed negative charges

SummaryCellulose acetate-nitrate filters were saturated with hydrophobic solvent and interposed between various aqueous solutions. The membranes thus formed are cation permselective. The discrimination between a monovalent cation such as K+ and the alkaline earth group divalent cations is very sharp. The discrimination ratio is at least a few thousand times in favor of the monovalent cation. A major part of this discrimination is caused by the very low mobility of the divalent cation within the membrane compared with that of the monovalent cation. The remainder of the discrimination is caused by the selectivity of the membranes which prefer monovalent to divalent cations. There is a clear discrepancy between Ba++ diffusibility and mobility within, the membrane. This implies that Ba++ may move within the hydrophobic membrane as a neutral complex. Some similarity with natural biological membranes is indicated.

The Modification by Biliproteins of Intensity and Direction of Electron Flow Across Chlorophyll-Containing Membranes

Photosensitive phenomena in artificial bileaflet membranes containing photosynthetic pigments are described and analyzed in terms of a potential model for the primary events in photo-synthetic energy storage. Illumination of a bilayer membrane interposed between aqueous solutions of different redox potential results in establishment of an electric current across the membrane. It is suggested that absorption of photons by the chlorophyll in the bilayer allows electron transfer across the membrane. This process differs from photosynthetic processes in two important aspects: the quantum efficiency of the process in the synthetic membrane is very low and energy is dissipated rather than stored. When biliproteins (extrinsic membrane proteins from blue-green and red algae) are introduced into the system, the resulting membranes exhibit two important properties: an increase by two- to threefold of the quantum efficiency of the electron transfer process and an abilty to direct the flow of electrons even against a pre-existing potential gradient, so that energy storage is accomplished. The observations are analyzed in terms of a model to explain the processes relevant to photosynthesis and the trivial process of degradation of the redox gradient.