R.P. Van Hoeven

Lipid structural order parameters (reciprocal of fluidity) in biomembranes derived from steady-state fluorescence polarization measurements

Abstract This paper presents an interpretation of fluorescence polarization measurements in lipid membranes which are labelled with the apolar probe 1,6-diphenyl-1,3,5-hexatriene. The steady-state fluorescence anisotropy, r S , is resolved into a fast decaying or kinetic component, r f , and an infinitely slow decaying or static component, r ∞ . The latter contribution, which predominates in biological membranes, is exclusively determined by the degree of molecular packing (order) in the apolar regions of the membrane; r ∞ is proportional to the square of the lipid order parameter. An empirical relation between r S and r ∞ is presented, which is in agreement with a prediction based on a theory of rotational dynamics in liquid crystals. This relation enabled us to estimate a lipid structural order parameter directly from simple steady-state fluorescence polarization measurements in a variety of isolated biological membranes. It is shown that major factors determining the order parameter in biomembranes are the temperature, the cholesterol and sphingomyelin content and (in a few systems) the membrane intrinsic proteins.

Studies on plasma membranes

SummaryPlasma membranes were isolated from rat and mouse livers, one rat hepatoma (and its subline) and two mouse hepatomas, and their lipid class compositions were determined. Lipids accounted for 30 to 35% of the dry weight of the membranes of livers and mouse hepatomas, and for 45% in the case of rat hepatoma-subline. Of the total lipids of rat-liver plasma membranes, 60% consisted of phospholipids, the corresponding values for mouse-liver and rat-hepatoma plasma membranes amounting to 55% and for both mouse-hepatoma plasma membranes to about 50%. The free cholesterol and cholesteryl ester contents of all hepatoma plasma membranes were significantly increased as compared with normal. Evidence is presented that the increase of free cholesterol was not a preparative artefact. The major phospholipid classes in all plasma membranes were phosphatidyl choline, sphingomyelin, phosphatidyl ethanolamine and phosphatidyl serine. The relative proportions in each plasma membrane species could differ appreciably, the mouse- and rat-liver membranes showing the closest resemblance. Possible reasons for (a) the higher level of phosphatidyl serine as compared with published values, and (b) the wide divergencies which may be found among the phospholipid profiles of rat-liver plasma membranes reported by other authors, are presented. Cardiolipin was absent from liver plasma membranes, but some could be found in the hepatoma membranes due to mitochondrial contamination. No consistent phospholipid profile characterized hepatoma as distinct from liver plasma membranes, nor did the hepatoma data-including plasmalogens-resemble the few available data on other hepatomas.

Phospholipid unsaturation and plasma membrane organization

A comparison has been made between the unsaturation of plasma-membrane phospholipids,present in the human erythrocyte, rat liver, mouse liver and a rapidly growing rat hepatoma. Of the double bonds present in the hydrocarbon chains of the membrane phospholipids,onethird is contributed by sphingomyelin plus phosphatidyl choline and the remainder by phosphatidyl serine, ethanolamine and inositol. Assuming that the phospholipids are asymmetrically distributed in the two leaflets of the bilayer in general, the consequences of this asymmetry in combination with cholesterol content and fatty acid distribution on plasma membrane organization and function are discussed. It is suggested, that the organizational disposition of plasma membrane components other than phospholipids is at least related if not dependent upon the latters asymmetric distribution in the bilayer.

Studies on plasma membranes. XXII. Fatty acid profiles of lipid classes in plasma membranes of rat and mouse livers and hepatomas.

Plasma membranes were isolated from rat and mouse livers, a transplanted rat hepatoma, two transplanted mouse hepatomas and spontaneous mouse hepatomas. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, sphingomyelin, lysophosphatidylcholine, free fatty acids and triglycerides were separated and their fatty acid profiles determined. The various lipid classes of rat and mouse liver plasma membranes each demonstrated more or less specific fatty acid profiles. The number of double bonds decreased in the order: phosphatidylserine greater than or equal to phosphatidylethanolamine greater than phosphatidylinositol greater than phosphatidylcholine greater than or equal to sphingomyelin and lysophosphatidylcholine. Small species differences were noted in most lipid classes. A marked sex difference was observed in sphingomyelin of mouse liver membranes but in none of the phospholipids of rat liver membranes. The increased cholesterol content of all hepatoma versus liver plasma membranes was accompanied by a decrease of fatty acyl poly-unsaturation in most lipid classes of the rat hepatoma but not of the mouse hepatoma membranes. The fatty acid profiles of the mouse hepatoma membranes deviated much less from those of mouse liver than did the pattern of rat hepatoma versus rat liver. The results were discussed in relation to lipid fluidity of which fatty acyl unsaturation and cholesterol are the main parameters.

Fluorescence polarization measurements on normal and tumour cells and their corresponding plasma membranes.

Using 1,6-diphenyl-1,3,5-hexatriene as a probe, the degree of fluorescence polarization (P) at 25 degrees C of intact and disrupted cells and isolated plasma membranes were compared for a variety of systems. 1. Human erythrocytes, mouse thymocyte and leukemia cells, rat liver and hepatoma cells, and human and mouse milk fat globules displayed P values ranging from 0.300 to 0.120. 2. P values or probe labelling rates of intact and disrupted cells were similar. 3. As compared with whole or disrupted cells, the higher to much higher P values of plasma membranes isolated from the corresponding cells showed only a limited mutual variation. 4. delta P values, being the difference in P values between plasma membranes and whole cells were attributed to the extent to which endomembranes and non-membrane lipids contributed. Among these, triglycerides had the greatest relative effect. 5. Though a particular isolation procedure for plasma membranes may select for more rigid fragments, this effect is by far not sufficient to account for the observed delta P values. It is concluded that the fluorescence polarization technique with a lipophilic probe applied to whole cells represents a measure of the average fluidity of all lipids being present in a cell and thus does not exclusively monitor the cell surface membrane.

High-pressure liquid chromatography of glycosphingo-lipids (with special reference to gangliosides)

The analysis of mixtures of gangliosides from adult human or bovine brain, supplemented with Tay-Sachs ganglioside, and haematoside from dog erythrocytes by high-pressure liquid chromatography using a moving-wire detector system is described. The complete separation of six gangliosides within 40 min has been achieved, using silica as the stationary phase and acidified chloroform-methanol-water mixtures as the eluent on a 25-cm column. Neutral glycosphingolipids, viz., the major components from normal human erythrocytes, can be completely separated on the same column, using non-aqueous and non-acidic eluents. It is shown that the methods described are useful for both analytical and (micro)-preparative purposes.

Monoclonal Antibodies Against Human Milkfat Globule Membranes Detecting Differentiation Antigens of the Mammary Gland

Abstract Monoclonal antibodies of mouse origin were raised against antigens of highly purified human milkfat globule membranes. Five groups of antigens, Mam-1 to Mam-5, could be distinguished based on immunoprecipitation followed by determination of the molecular weights, their presence or absence from milk proteins and their tissue distribution in normal and malignant mammary glands.