Optical-rotatory-dispersion studies of compounds related to cholesterol in liposomes and the membranes of erythrocyte ‘ghosts’

Abstract

1. Steroid molecules containing the alpha,beta-unsaturated oxo group in various positions were incorporated with egg phosphatidylcholine into liposomes and into human erythrocyte membranes. 2. The liposomes formed contained 0.3-0.94mol of steroid/mol of phospholipid and the steroids replaced 19-76% of the erythrocyte membrane sterol. 3. The optical rotatory dispersion (o.r.d.) spectra of the steroids in these structures were compared with those obtained in solvents of different polarity. 4. The o.r.d. spectra of cholesta-4,6-dien-3-one and 3-hydroxycholest-3-en-2-one in liposomes resembled those obtained with polar solvents such as ethanol or triethyl phosphate-water (1:1, v/v). 5. The o.r.d. spectra of 3-hydroxycholest-7-en-6-one and 3-hydroxycholest-5-en-7-one in liposomes resembled those obtained with moderately polar solvents such as dioxan. 6. The o.r.d. spectrum of 3-hydroxycholest-8(14)-en-15-one in liposomes resembled those obtained with non-polar solvents such as cyclohexane. 7. 3-Hydroxycholest-3-en-2-one did not exchange with erythrocyte membrane cholesterol, but the other steroids did do so and the o.r.d. spectra of the membranes containing them closely resembled those obtained with liposomes. 8. From the results, the position of sterol molecules with respect to the phospholipid molecules in liposomes and membranes of human erythrocyte ;ghosts' can be deduced.

The rate of cholesterol ‘flip‐flop’ in lipid bilayers and its relation to membrane sterol pools

“…Egg lecithin preparation, lipid extraction, estimation of phospholipid and cholesterol were as described previously [6,7]. Low density lipoproteins were obtained from human plasma by ultracentrifugation [ 81 and dialysed against 0.15 M phosphate buffer, pH 7.4.…”

“…Sterophenol was prepared from cholesta-1,4, 6-trien-3-one [9] and estimated spectrophotometritally in ethanol solution (A,,, 285 nm, E 1995). Liposomes were prepared as before [7] except that only three one minute periods of sonication interrupted by 30 second cooling periods were used. After centrifugation at 105 000~ for 20 min, the suoematant liposome dispersion consisted almost entirely of single-shelled vesicles [lo] .…”

Conformational Changes in Fibrous Elastin Due to Calcium Ions

“…Transfer or exchange of cholesterol from liposomes to erythrocyte membranes has been shown to occur under "physiological" conditions [18,19] and there is little reason to doubt the existence of an equilibrium between cholesterol absorbed on elastin fibres and cholesterol in the blood circulation even though the rate of exchange may be slow. It should be pointed out, however, that the experiments described here were performed in neutral salt solutions containing 2-9 mg/ml of taurodeoxycholate as a carrier to endow cholesterol solubility, and thus the properties of the solvent may not be comparable with those of the extracellular fluid in artery walls.…”

Use of fluorescent probes in the study of phospholipid–sterol bilayers

“…Egg phosphatidylcholine, 3-hydroxycholest-3-en-2-one and 1-methyl-10-norcholesta-1,3,5(10)-trien-3-ol were made as before (Green et al, 1973;Smith & Green, 1974).…”

“…Such molecules are of interest as components of bilayers both because they can be used as structural probes (Green et al, 1973) and because they may be able to modify the properties of liposomes injected in vivo (Wharton & Green, 1978).…”