Effect of phloretin on the permeability of thin lipid membranes.

Abstract

Phloretin greatly increases cation conductances and decreases anion conductances of membranes treated with ion carriers (nonactin, valinomycin, carbonyl-cyanide-m-chlorophenylhydrazone [CCCP], and Hg(C6F5)2) or lipophilic ions (tetraphenylarsonium [TPhAs+] and tetraphenylborate [TPhB-]). On phosphatidylethanolamine membranes, 10-4 M phloretin increases K+-nonactin and TPhAs+ conductances and decreases CCCP- and TPhB- conductances 103-fold; on lecithin:cholesterol membranes, it increases K+-nonactin conductance 105-fold and decreases CCCP- conductance 103-fold. Similar effects are obtained with p- and m-nitrophenol as 10-2 M. These effects are produced by the un-ionized form of phloretin and the nitrophenols. Phloretin, which possesses a large dipole moment, probably adsorbs and orients at the membrane surface to introduce a dipole potential of opposite polarity to the preexisting positive one, thus increasing the partition coefficient of cations into the membrane interior and decreasing the partition coefficient of anions. (Phloretin may also increase the fluidity of cholesterol-containing membranes; this is manifested by its 2- to 3-fold increase in nonelectrolyte permeability and its asymmetrical effect on cation and anion conductances in cholesterol-containing membranes). Phloretin''s inhibition of chloride, urea and glucose transport in biological membranes may result from the effects of these intense interfacial dipole fields on the translocator(s) of these molecules.