Systems approach to study of solute transport across membranes using supension cultures of mammalian cells IV: Uptake and release kinetics of sterols

The kinetics of uptake and release of desmosterol, cholesterol, and β-sitosterol by Burkitt lymphoma cells at 26°, pH 7.3, and isoosmotic conditions were quantitatively and mechanistically interpreted. The data are in agreement with the model involving the passive transport of the unbound sterol across the ratedetermining plasma membrane, with rapid distribution within the heterogeneous cell interiro. Effective permeability (P_e) and partition (K_e) coefficients of the sterols were inversely proportional to the serum concentration in the external media due to sterol-serum binding. These results are consistent with the mechanism in which only the unbound solute in the external solution participates in the membrance transport process. At al serum levels, P_e and K_e increased with increasing sterol polarity: desmosterol > cholesterol > β-sitosterol.