Distribution and partitioning of cholesterol and beta-sitosterol in micellar bile salt solutions.

The distribution of cholesterol and beta-sitosterol in micellar bile salt solutions was studied using an ultrafiltration technique to separate micellar aggregates from monomers present in the intermicellar aqueous phase. The partitioning and the rates of influx across the brush-border membrane of these two sterols from micellar solutions were determined using polyethylene discs and everted jejunal sacs, respectively. Micellar solubilities of cholesterol and beta-sitosterol were not significantly different, whereas the monomer concentration of beta-sitosterol in the intermicellar aqueous phase was significantly lower than that of cholesterol [0.21 +/- 0.02 microM for beta-sitosterol and 25.0 +/- 2.71 (SE) microM for cholesterol, P less than 0.001]. There was no difference in the partitioning nor was there a difference in the rates of influx between cholesterol and beta-sitosterol from micellar solutions containing a similar amount of the two sterols. It is concluded that monomer concentration of beta-sitosterol in the intermicellar aqueous phase is extremely low compared with that of cholesterol, but their partitioning and rates of influx across the membrane are similar. This is primarily attributable to a much higher membrane/monomer partition coefficient of beta-sitosterol than cholesterol and to a direct interaction between micelle and membrane.