Studies on the regulation of sterol synthesis in lymphocytes from atherosclerotic-susceptible and -resistant pigeons. Efflux of sterols and membrane fluidity

Fibrillar amyloid beta-protein forms a membrane-like hydrophobic domain

Microviscosity of the biological membranes is determined by measuring the fluorescence polarization of diphenylhexatriene (DPH). DPH, a hydrophobic probe, has negligible fluorescence in the solution. When DPH is incorporated into the membrane, it is localized in the membrane hydrophobic core and fluoresces strongly. We report here that DPH also fluoresces in the presence of fibrillar Abeta (fAbeta). However, it does not fluoresce when it is added to the soluble Abeta (sAbeta). DPH inserts into Abeta fibrils in a time-dependent manner, and upon centrifugation, it is sedimented along with fibrils. The steady state fluorescence polarization of DPH with fAbeta1-40 and fAbeta 1-42 was 0.4592 and 0.4898 respectively. These results suggest that fAbeta (but not sAbeta) forms a hydrophobic domain similar to that of membrane.

Membrane fluidity and cholesterol in thymus and spleen cells from mice treated with immunomodulatory drugs

We have used spin labeling, fluorescence polarization, and chemical analysis to characterize membrane properties of thymocytes from mice treated with immunomodulatory drugs. The number of thymocytes was reduced 90-95% by treatment of 6-9 week old mice with hydrocortisone acetate (HCA) or methylprednisolone (both 125 mg/kg) or with cyclophosphamide (250 mg/kg). Electron spin resonance (esr) examination of thymocytes labeled with 5-nitroxyl stearic acid indicated that the membranes of cells remaining after treatment with any of these drugs were more rigid than those from saline-treated controls. The total cholesterol/phospholipid (C/PL) molar ratio of the HCA-resistant thymocytes was twice that of the control mice. Treatment of mice with other immunomodulatory drugs, cyclophosphamide, cytosine arabinoside (Ara-C), 2-amino-5-bromo-6-phenyl-4-(3H)-pyrimidinone (ABPP) and 15(S)-methyl prostaglandin E1 (15(S)-methyl PGE1), also altered the C/PL ratio in thymocytes and, in some cases, in spleen cells. Fluorescence polarization measurements of thymocytes labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH) did not reveal the differences between cells from HCA-and saline-treated mice that were detected by spin labeling and chemical analysis. Our results indicate that the greater rigidity detected by spin labeling of hydrocortisone-resistant thymocytes may be due, at least in part, to greater membrane cholesterol content. Of the methods employed, chemical analysis was the most sensitive in revealing drug-induced alterations in thymocyte populations.

Phospholipid methylation of kidney cortex brush border membranes. Effect on fluidity and transport

Exposure of intact brush border membrane vesicles of hog kidney cortex to cholesterol oxidase resulted in 24% oxidation of membrane cholesterol compared with more than 95% oxidation of cholesterol in lipids isolated from membranes, showing that cholesterol is asymmetrically distributed in membranes. Phospholipase C, hydrolyzed 76% of phosphatidylcholine and 10-12% phosphatidylethanolamine while phosphatidylserine was not hydrolyzed, thus indicating that majority of phosphatidylcholine is present on the outer surface of these vesicles while phosphatidylethanolamine and phosphatidylserine are present on the inner surface. Methylation of phospholipids in brush border membrane with S-adenosyl-[methyl-3H]methionine resulted in the formation of phosphatidyl-N-monomethylethanolamine, phosphatidyl-N,N]dimethylethanolamine and phosphatidylcholine from endogenous phosphatidylethanolamine. The Km for S-adenosylmethionine was 1.10(-4) M with an optimum pH 9.0 for the formation of all three methyl derivatives. Mg2+ was without any effect between pH 5 to 10. Addition of exogenous mono- and dimethylphosphatidylethanolamine derivatives enhanced methyl group incorporation by 4-5-fold as compared to the addition of phosphatidylethanolamine. The conversion of endogenous phosphatidylethanolamine to phosphatidyl-N-monomethylethanolamine or addition of exogenous phosphatidylmonomethylethanolamine to brush border membrane did not result in a change in bulk membrane fluidity as determined by fluorescence polarization of diphenylhexatriene. Methylation of phosphatidylethanolamine in brush border membrane did not affect the Na+-dependent uptake of either D-glucose or phosphate, although the accessibility of cholesterol in membrane to cholesterol oxidase was diminished by 21%, presumably due to altered flip-flop movement of cholesterol in the membrane.