Ross P. Holmes

25-Hydroxysterols increase the permeability of liposomes to Ca2+ and other cations

25-Hydroxycholesterol and 25-hydroxy vitamin D-3 increased the permeability of liposomes to Ca2+ measured by the arsenazo III encapsulation technique. This effect was sensitive to the lipid composition of the membrane, with changes that decreased the motional freedom of phospholipid acyl chains decreasing Ca2+ permeability. The greatest permeability was observed with the zwitter-ionic phospholipids, phosphatidylcholine and phosphatidylethanolamine, whereas the acidic phospholipids, phosphatidylinositol and phosphatidylserine, depressed Ca2+ permeability. The effect was not specific for Ca2+. Other divalent cations were translocated in the order Mn2+ greater than Mg2+ = Ca2+ much greater than Sr2+ = Ba2+. The permeability of liposomes to the monovalent cation, Na+, was also substantially increased. The effect did not appear to be due to ionophoretic properties of the sterols, and it is suggested that perturbation of the membranes by the polar 25-hydroxyl group may play a role in increasing membrane permeability.

THE EFFECT OF MEMBRANE LIPID COMPOSITION ON THE PERMEABILITY OF MEMBRANES TO Ca2

An important property of membranes is their impermeability to Ca” and other cations. This has most clearly been demonstrated in studies with phosphatidylcholine liposomes.’ Ca2+, however, is an essential cellular component that is required for many cellular functions. Furthermore, fluctuations in cytoplasmic levels of Ca2+ are believed to regulate many intracellular processes.2 Mechanisms must therefore exist for Ca2+ to cross cellular membranes to serve its regulatory function and to meet growth requirements. Cellular mechanisms known to exist are illustrated in FIGURE 1. (A) denotes active transport systems, which have been identified in mitochondria, the endoplasmic reticulum, and plasma membranes. The requirement for energy to transport Ca” is clearly evident for the plasma membrane system. A concentration gradient of lo4 across this membrane coupled with a potential difference of approximately 50 mV would favor a massive Ca2+ influx if Ca2+ was able to freely distribute across the membrane. Similar systems may exist in the nuclear envelope and in intranuclear vesicles3 where they have been proposed t o regulate Ca’’ fluxes during mitosis. Mechanisms must exist for Ca’+ to be released from the intracellular organelles that sequester Ca2+, but they have not been included in FIGURE 1 as their nature is uncertain. (E) represents Na+-Ca’’ exchange that has been identified in the plasma membranes of excitable cells but not as yet in non-excitable cells.4 Its stoichiometry is uncertain and it is believed to act to extrude Ca2+ from the cell, although in vitro it can be manipulated to operate in either direction

The analysis of 25-hydroxycholesterol in plasma and cholesterol-containing foods by high-performance liquid chromatography

A method for the analysis of 25-hydroxycholesterol by high-performance liquid chromatography (HPLC) in a wide range of materials is described. Lipid extracts were initially purified on octadecyl silicic acid cartridges and by reversed-phase HPLC before quantitation by HPLC on a silicic acid column. The reproducibility of the method was confirmed by the analysis of plasma from rats fed 25-hydroxycholesterol. 25-Hydroxycholesterol was not detected in lard, cream, fresh egg yolk or spray-dried egg yolk powder. It was detected in egg yolk powder after heating at 110 degrees C for 4 days and its authenticity was confirmed by mass spectrometry.

Irreversible inhibition of water transport in erythrocytes by fluoresceinmercuric acetate

Abstract Fluoresceinmercuric acetate (FMA) is approximately 5 times more potent than p-chloromercuribenzene sulfonate (PCMBS) in inhibiting water transport across human erythrocyte membranes. Half maximal inhibition occurred at approximately 60 m. While cysteine fully reversed the inhibition induced by p-chloromercuribenzene sulfonate it had no effect on the inhibition induced by fluoresceinmercuric acetate. A comparison of the structures of fluoresceinmercuric acetate and p-chloromercuribenzene sulfonate suggests that either there are two sulfhydryl groups in close proximity or the larger aromatic ring structure of fluoresceinmercuric acetate causes tighter binding to one sulfhydryl group at the active site.

The saturation and isomerization of dietary fatty acids and the respiratory properties of rat heart mitochondria

Weanling rats were fed semi-purified diets containing 15% by weight of either corn oil, a high oleic acid safflower oil, lard or hydrogenated soybean oil. Significant changes in the fatty acid composition of heart mitochondrial preparations were induced by these dietary fats. Despite these changes in membrane composition, no effects on the respiratory properties of the mitochondria were observed. These results suggest that mitochondrial membranes adapt to changes in dietary fatty acids in a way which prevents changes in their functional properties.

The effect of the saturation and isomerization of dietary fatty acids on the osmotic fragility and water diffusional permeability of rat erythrocytes

Weanling rats were fed semi-purified diets containing 15% by weight of either corn oil, a high oleic acid safflower oil, lard or hydrogenated soybean oil. Significant changes in the fatty acid composition of erythrocytes were induced by these dietary fats. The compositional changes did not effect water diffusional permeability, but did affect their osmotic fragility. An increased fragility appeared to be associated with an increased octadecenoate content of the membranes.

Modifications of Human Erythrocyte Membranes and Their Effect on Water Permeability Studied by a Nuclear Magnetic Resonance Technique

Although the process of water transport across biological membranes is of considerable importance for many physiological processes and various hypotheses have been proposed and investigated, the mechanism controlling water movement has not been fully elucidated. Because of its simple structure, lacking internal membranes, the red blood cell is ideally suited for studying water permeability.