The activity of 5'-nucleotidase in liver plasma membranes is affected by the increase in bilayer fluidity achieved by anionic drugs but not by cationic drugs.

The 5'-nucleotidase activity of rat liver plasma membranes could be selectively modulated by the anionic drugs phenobarbital and pentobarbital, whereas the corresponding activity of a Lubrol-solubilized preparations remained unaltered. The perturbation in the outer half of the bilayer induced by phenobarbital, which lead to a depression in the high temperature onset of the lipid phase separation occurring in this half of the bilayer, concomitantly lowered the break temperature in Arrhenius plots of 5'-nucleotidase activity from 28 degrees C to 16 degrees C. The stimulation of the membrane-bound activity achieved by low anionic drug concentrations was attributed to a preferential fluidization of the outer half of the bilayer. Contrarily, the cationic drugs prilocaine and carbocaine, when tested over agent concentrations that dramatically increase the fluidity of the inner half of the bilayer, achieved no selective effects on the membrane-bound enzyme. Prilocaine (10 mM) was previously found to induce a lipid phase separation at 11 degrees C that was attributed to the lipids of the internal (cytosol-facing) half of the bilayer, but had no effect on the onset of the lipid phase separation occurring at 28 degrees C. Since Arrhenius plots of 5'-nucleotidase activity in the presence of 10 mM prilocaine concentrations demonstrated only the single break at 28 degrees C, we suggest that prilocaine is unable to selectively perturb the enzyme because this cationic drug preferentially interacts with the acidic phospholipids residing in the inner half of the bilayer. The activity of the ectoenzyme 5'-nucleotidase in rat liver plasma membranes appears to be regulated by the external half of the bilayer only. These results support the view that independent modulation of he fluidity or chemical constituents of each half of the bilayer can distinctly affect the activity of proteins that are themselves asymmetrically orientated within the bilayer.