Effect of an aqueous phase on the solubility of cholesterol in an oil phase.

In the absence of water, the solubility of cholesterol in triolein at 21 degrees C was 2.8%. When water was added to the system, the solubility of cholesterol in the oil phase decreased to 1.9%, and cholesterol monohydrate precipitated. The precipitation of the sterol evidently resulted from the excess concentration of the surface-active cholesterol at the interface, allowing the rapid interaction of water with cholesterol and the resulting formation of cholesterol monohydrate with its attendant lower energy and less soluble crystalline lattice. The ternary phase diagram for the system cholesterol--triolein--water, modified to include cholesterol monohydrate formation with the consequent decrease in sterol solubility; differs from the previously reported phase diagram. Other cholesterol--oil--aqueous systems related to biologically important lipids were studied. Cholesteryl oleate was more soluble than cholesterol in triolein (23% at 21 degrees C), and this value did not decrease when water was present. Water caused cholesterol to precipitate from cholesteryl linoleate at 37 degrees C. Thus crystalline cholesterol may be present in lipids found in atherosclerotic plaques at a lower concentration of free cholesterol than had been predicted previously. In another experiment, a micellar taurocholate solution precipitated cholesterol from triolein and from corn oil. These effects of aqueous systems suggest the possibility of cholesterol precipitation from dietary fat when it becomes mixed with water in the diet or stomach, or with the micellar phase in the intestine. Plant sterols were precipitated also from oil solutions by an aqueous phase. Water-induced sterol precipitation is a phenomenon that could occur in a variety of biological systems, and may be applicable to sterols in general.