Membrane fluidization by ether, other anesthetics, and certain agents abolishes P-glycoprotein ATPase activity and modulates efflux from multidrug-resistant cells

The anesthetics benzyl alcohol and the nonaromatic chloroform and diethyl e ther, abolish P-glycoprotein (Pgp) ATPase activity in a mode that does not fit classical competitive, noncompetitive, or uncompetitive inhibition. At concentrations similar to those required for inhibition of ATPase activity, these anesthetics fluidize membranes leading to twofold acceleration of do xorubicin flip-flop across Lipid membranes and prevent photoaffinity labeli ng of Pgp with [I-125] -iodoarylazidoprazosin. Similar concentrations of et her proved nontoxic and modulated efflux from Pgp-overexpressing cells. A s imilar twofold acceleration of doxorubicin flip-flop rate across membranes was observed with neutral mild detergents, including Tween 20, Nonidet P-40 and Triton X-100, and certain Pgp modulators, such as verapamil and proges terone. Concentrations of these agents, similar to those required for membr ane fluidization, inhibited Pgp ATPase activity in a mode similar to that o bserved with the anesthetics. The mode of inhibition, i.e. lack of evidence for classical enzyme inhibition and the correlation of Pgp ATPase inhibiti on with membrane fluidization over a wide range of concentrations and struc tures of drugs favors the direct inhibition of Pgp ATPase activity by membr ane fluidization. The unusual sensitivity of Pgp to membrane fluidization, as opposed to acceleration of ATPase activity of ion transporters, could fi t the proposed function of Pgp as a 'flippase', which is in close contact w ith the membrane core.