The Amphiphilicity of Allorcits and the Rate of ATP Hydrolysis and Flopping by P-Glycoprotein

Abstract

We investigated how the amphiphilicity of detergents influences the rate of ATP hydrolysis by P-glycoprotein. To this purpose we used 34 electrically neutral and cationic detergents with different head groups and alkyl chain lengths and thus varied amphiphilicity. The free energy of detergent partitioning into the lipid membrane (dominated by the alkyl chain) and the free energy of detergent binding from the membrane to the transporter (due to the polar head group) were determined. For seven of these detergents the rate of ATP hydrolysis (Log scale) decreased linearly with increasing affinity from water to the transporter as observed previously for typical P-glycoprotein allocrits. Although, structurally closely related the remaining 27 detergents significantly reduced the rate of ATP hydrolysis. Inhibition of the P-glycoprotein-ATPase due to membrane disordering could be excluded based on order parameter measurements of deuterated lipid bilayers in the presence of relevant detergent concentrations using D-NMR spectroscopy. An optimal rate of ATP hydrolysis was observed only if the two free energies of binding were in appropriate proportion to each other, i.e., if the ratio of the two free energies of binding was slightly above three for cationic, and somewhat below three for neutral detergents. Lower or higher ratios inhibited the P-glycoprotein-ATPase activity and effective allocrit flopping.