Self-association of Cholesterol in Aqueous Solution

Abstract

Cholesterol has a maximum solubility in aqueous solutions of 1.8 jug/ml or 4.7 ,M. It undergoes a thermodynamically reversible self-association with a critical micelle concentration of 25 to 40 nM at 250. The cholesterol micelle is heterogeneous in size, probably rodlike in shape, and stabilized by an unusually high interaction energy between the aggregated monomers.A critical problem in biochemistry today is the elucidation of the molecular organization of lipid and protein components in biological membranes and serum lipoproteins. Not only is a definition of the structure itself essential, but also an explanation in thermodynamic terms for its existence, i.e., free energies of association among the various components. However, any such investigation requires prior knowledge of the physical properties of the components themselves in aqueous solvents.Lipids, for example, are amphiphiles that undergo monomeraggregate equilibria. Just as it was essential to define these equilibria in thermodynamic terms in order to interpret binding of simple detergents to serum albumin (1, 2), so the same information is required to determine the modes of association of biological lipids with membrane proteins and serum lipoproteins. Numerous structural investigations of pure phospholipid bilayers and of the incorporation of cholesterol into these bilayers have been published (3-5). However, only a single phospholipid system, dipalmitoyl phosphatidylcholine, has been studied from the standpoint of monomer-micelle equilibrium (6). Cholesterol-water solutions have been completely neglected despite the ubiquitous nature of this neutral lipid in eukaryotes.We report here the critical micelle concentration of cholesterol in water and the formation of a heterogeneous selfaggregate in which the forces of interaction are stronger than those usually found in micellar systems. (Beckman Instruments). Scintillation fluid containing 2 liters of toluene, 1 liter of Triton X-100, 8.0 g of 2,5-diphenyloxazole, and 0.4 g of 1,4-bis-2-(5 phenyl-oxazolyl)-benzene was used. The aqueous sample was added to 15 ml of scintillation fluid; the mixture was equilibrated for 4 hr in the dark and counted to 1.5% error. The efficiencies of counting over a range of 0-0.1 ml of water were determined by standardization with [3H]toluene of 3.12 X 106 dpm/ml. A sample of 50-100 jl had a counting efficiency of 40%. All counts were corrected for background, efficiency, and quenching before conversion to concentration units.Critical micelle concentrations were determined by rates of dialysis by use of 10 ml of water in a Visking dialysis bag (Union Carbide Corp.) suspended in a glass vessel containing 15 or 25 ml of cholesterol solution. The apparatus was designed so that a closed system existed except at the time of sampling. The solution was mixed by rotation at 250. The dialysis tubing was brought to a boil in water, rinsed extensively, and used within 1 day.Lengths of micellar aggregates were estimated by use of

Ohr plays a central role in bacterial responses against fatty acid hydroperoxides and peroxynitrite

“…However, we cannot exclude the possibility that ChOOH could not access the XfOhr active site, because this molecule can form micelles. Indeed, the parental compound (cholesterol) presents very a low CMC (25-40 nM) (36). On the other hand, because Triton X-100 and Tween-20 at least partly solubilize ChOOH (30), our results ( (Fig.…”

Lipid transfer protein binding of unmodified natural lipids as assessed by surface plasmon resonance methodology

“…In the case of cholesterol, it was necessary to work at concentrations above its reported aqueous CMC of 30-40 nM [46] in order to detect SPR responses (Fig. 6A).…”

NPC2, the Protein Deficient in Niemann-Pick C2 Disease, Consists of Multiple Glycoforms That Bind a Variety of Sterols

“…The relatively strong binding of cholesterol sulfate is likely to arise from its solubility properties, rather than its intrinsic affinity for NPC2. Haberland and Reynolds (35) have reported that cholesterol has a strong propensity to aggregate in aqueous solution, with a critical micelle concentration of ϳ25 nM, and have discussed how binding of cholesterol to a protein is a balance between the free energy gained from association with that protein and the free energy gained from self-association. It is likely that cholesterol sulfate is less prone to aggregation than cholesterol itself and that this alone may explain the greater apparent affinity of the negatively charged ligand for NPC2.…”