Effect of polysorbate 60 on interphase transport of cholesterol

Interphase cholesterol transport was investigated at 24 +/- 1 degrees in a stirred diffusion cell and in various oil-in-water emulsions. Cholesterol uptake by vegetable oil from a cholesterol-surfactant-rich aqueous phase was extremely slow in the stirred cell; no measurable transport had occurred after 500 hr. Cholesterol transport in oil-in-water emulsions following dilution with a cholesterol-surfactant-rich aqueous phase was much faster due to the greatly increased interfacial area available for mass transfer. Equilibration half-lives, t(50), varied from 2.02 to 28.1 hr. Variations in the t(50) were due to: (a) differences in the mean oil droplet diameter among various emulsions, and (b) differences in cholesterol-polysorbate 60 micelle sizes among various dilution media. When polysorbate 60 was omitted from the dilution medium, transport occurred in a two-stage process. In the first stage, transport was extremely rapid, with the t(50) less than 30 sec; in the second stage, transport was comparable to previous emulsion rates, with the t(50) varying from 7.9 to 8.1 hr. The significance of this two-stage transport to mechanisms of interphase cholesterol transport is briefly discussed.