THE FUNCTION OF BILE SALTS IN FAT ABSORPTION. THE SOLVENT PROPERTIES OF DILUTE MICELLAR SOLUTIONS OF CONJUGATED BILE SALTS

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Abstract
Micellar solubilization by bile salts of fatty acids and monoglycerides has been proposed as an important step in the absorption of dietary fat. The solvent properties of dilute (less than 20 mM) bile salt solutions in vitro have been studied for 2 model solutes a non-polar solute, trans-azobenzene (azobenzene), and a polar or amphiphilic solute, glycerol 1-mono-oleate (mono-olein). Six conjugated bile salts and a mixture simulating the bile salt present in human-intestinal content were compared with 3 typical anionic detergents. The saturation ratio (moles of micellar azobenzene of mono-olein/mole of micellar bile salt or detergent) was calculated. For both solutes, dihydroxy conjugates had a lower critical micellar concentration and a higher saturation ratio than the trihydroxy conjugates. The bile salt micelle had a lower saturation ratio for azobenzene but a much higher saturation ratio for mono-olein than the typical anionic deter gentmicelle. The solvent power of the bile salt micelle for azobenzene was increased greatly by the addition of mono-olein. There was no effect of pH on the solubility of azobenzene or mono-olein in bile salt solutions. The micellar solubility of azobenzene rose sharply, then levelled with increasing Na+ ion concentration. The rapid increase was chiefly attributable to an increased saturation ratio. The solubility of mono-olein in bile salt solutions increased slightly with increasing Na+ ion concentration. Azobenzene was less soluble at 23[degree] than at 37[degree] in bile salt solutions. This decrease was caused by a fall in its solubility in buffer and a lower saturation ratio. The former factor was dominant at low bile salt concentrations, the latter at high bile salt concentrations. Mono-olein had an identical solubility at 23[degree] and 37[degree] in bile salt solutions. Bile salts were not precipitated from solution by the addition of Ca2+ ions, under conditions where typical anionic detergents were.