Mechanism and rate of permeation of cells by XXXX .

@entity2556 @entity3014 @entity2434 @entity2555
The principal mechanism of cellular uptake of @entity3014 and other @entity2555 ( @entity2556 ) from lipoproteins into cells is spontaneous transfer through the aqueous phase (Plant, A. L., Benson, D.M., and Smith, L.C. (1985) J. Cell Biol. 100, 1295-1308). Cellular uptake of @entity3014 from low density lipoproteins followed first-order kinetics with a rate constant that was independent of the relative lipoprotein concentrations or cell number but which was 2 orders of magnitude smaller than the rate constant for @entity3014 desorption from low density lipoproteins. Moreover, identical rate constants for cellular uptake of @entity3014 were observed when the donor vehicle was high density lipoproteins, very low density lipoproteins, or single bilayer @entity2434 vesicles, even though rate constants for @entity3014 transfer from these donor vehicles differed by 10-fold. When @entity2434 vesicles containing @entity3014 and a nontransferable fluorescence quencher were mixed with cells in a stopped-flow system, two kinetic components were distinguished: a fast component with a rate constant corresponding to that measured for transfer of @entity3014 out of vesicles, followed by a much slower component, with a time course approximating that measured for cellular accumulation of @entity3014 by other techniques. Rate constants for desorption of a series of @entity2556 which contained different number of aromatic rings from @entity2434 vesicles differed over a 70-fold range. First-order rate constants for cell uptake of @entity3014 and five other @entity2556 of different molecular sizes had the same 70-fold range of values, but were 2 orders of magnitude smaller than their respective rate constants for desorption from single bilayer vesicles. In addition, activation energies for cell uptake were essentially identical to the respective activation energies for desorption of @entity2556 from @entity2434 vesicles, confirming the mechanistic similarity of the two processes.
