BEHAVIOR OF CHOLESTEROL AND SPIN-LABELED CHOLESTANE IN MODEL BILE SYSTEMS STUDIED BY ELECTRON-SPIN-RESONANCE AND SYNCHROTRON X-RAY

The behavior of mixed bile salt micelles consisting of sodium taurocho late, egg phosphatidylcholine, and cholesterol has been studied by ESR spin labeling and synchrotron x-ray scattering. Consistent with publi shed phase diagrams, pure and mixed bile salt micelles have a limited capacity to incorporate and, hence, solubilize cholesterol. Excess cho lesterol crystallizes out, a process that is readily detected both by ESR spin labeling using 3-doxyl-5 alpha-cholestane as a probe for chol esterol and synchrotron x-ray scattering, Both methods yield entirely consistent results. The crystallization of cholesterol from mixed bile salt micelles is indicated by the appearance of a magnetically dilute powder spectrum that is readily detected by Visual inspection of the ESR spectra. Both the absence of Heissenberg spin exchange and the obs ervation of a magnetically dilute powder spectrum provide evidence for the spin label co-crystallizing with cholesterol. In mixed bile salt micelles containing egg phosphatidylcholine, the solubility of cholest erol is increased as detected by both methods. With increasing content of phosphatidylcholine and increasing mole ratio cholesterol/phosphat idylcholine, the anisotropy of motion of the spin probe increases, The spin label 3-doxyl-5 alpha-cholestane is a useful substitute for chol esterol provided that it is used in dilute mixtures with excess choles terol: the cholesterol/spin label mole ratio in these mixtures should be greater than 100. Despite the structural similarity between the two compounds, there are still significant differences in their physico-c hemical properties. These differences come to the fore when cholestero l is totally replaced by the spin-label: 3-doxyl-5 alpha-cholestane is significantly less soluble in bile salt and mixed bile salt micelles than cholesterol and, in contrast with cholesterol, it interacts only very weakly, if at all, with phosphatidylcholine. The potential of the ESR method for detecting cholesterol crystal growth in human bile is discussed.