Bile formation and secretion control homeostatic mechanisms for eliminating cholesterol and tetrapyrrole molecules form the organism as well as absorption of dietary fat. Bile dysfunction causes several common diseases, including gallstones and cholestasis. This proposal employs biophysical rationale and physical-chemical methodology to further molecular understanding of the physical biochemistry of bile, its formation, secretion and functions. The PI and colleagues will design and study appropriate model systems and correlate the results with pathophysiological phenomena pertaining to the function and dysfunction of native systems. They will use I)novel flurocholesterol methodology, cryoelectron microscopy and electron energy-loss spectroscopy to elucidate physical-chemical pathways whereby cholesterol molecules are transferred from blood to liver cell and bile, ii) characterize interactions of bile salt molecules with sphingomyelin in micellar solutions and at interfaces related to cholesterol secretion, absorption and apoptosis, iii) determine the physical-chemical origin and pathophysiology of lipoprotein X in bile secretory failure, iv) define how phosphatidylcholine, cholesterol and calcium influence the physical- chemical state of natural conjugated bilrubins in model (bilrubin ditaurate) and native biles employing analytical ultracentrifugation and spectrophotometric techniques, v) measure the metastable and equilibrium solubilities of unconjugated bilirubin in modelbiles utilizing potentiometric titration and dissolution and correlate the information pathophysiologically with pigment-stone biles, vi) discover whether humans with ~black~ pigment gallstones have dysfunctional mutations of the ileal bile acid transporter gene. These objectives are designed to advance our understanding of physical chemistry of bile as ell as normal and abnormal movements of cholesterol and billirubin to and from the liver and alimentary tract. The systematic project should lead to new targets and strategies for prevention of pigment and cholesterol gallstone diseases as well as lipid transport abnormalities sin cholestasis.