SPONTANEOUS RATES OF SITOSTEROL AND CHOLESTEROL EXCHANGE BETWEEN PHOSPHOLIPID-VESICLES AND BETWEEN LYSOPHOSPHOLIPID DISPERSIONS - EVIDENCE THAT DESORPTION RATE IS IMPEDED BY THE 24-ALPHA-ETHYL GROUP OF SITOSTEROL

The rates of spontaneous movement of [4-C-14]sitosterol and [4-C-14)cholesterol between unilamellar vesicles prepared with different phospholipid-sterol molar ratios and with different classes of phospholipids and between lysophospholipid-enriched dispersions have been measured. The rates of desorption of sitosterol from dipalmitoylphosphatidylcholine, egg phosphatidylcholine, and sphingomyelin bilayers are lower than those of cholesterol by factors of 3-9 over the range of 1-50 mol % sterol at 50-degrees-C. The rate of movement of sitosterol from aqueous dispersions containing 1.5 mol % sterol and 50 mol % lysophosphatidylcholine is lower than that of cholesterol by a factor of 4 at 37-degrees-C. The additional van der Waals forces made possible by the 24-ethyl group or sitosterol and its nearest neighbors, together with a lower aqueous phase solubility because of the presence of the 24-alkyl group, are considered to contribute to the slower desorption of sitosterol from the lipid-water interfaces relative to cholesterol. Since cholesterol is carried by mixed micelles and phospholipid vesicles in vivo, our results suggest that the 24-alkyl group of the important plant sterols sitosterol and campesterol may play an important role in the phenomenon of selective uptake of exogenous cholesterol by intestinal cells from lipid surfaces. We also discuss the extent of spontaneous cholesterol exchange between vesicles at equilibrium in relation to results obtained in earlier studies.