Biochemical mechanism of aminoglycoside-induced inhibition of phosphatidylcholine hydrolysis by lysosomal phospholipases.

Abstract

Aminoglycosides such as gentamicin are hydrophilic, polycationic drugs which bind to negatively-charged phospholipid bilayers, inhibit the activities of the lysosomal enzymes involved in the degradation of the major phospholipids and cause, in kidney in vivo or in cultured cells, a lysosomal phospholipidosis. In the present study, we show that the hydrolysis of phosphatidylcholine induced in liposomes by lysosomal extracts at pH 5.4 in vitro is critically dependent on the negative charges carried by the bilayer. This hydrolysis, which is predominantly carried on by phospholipases A1 and A2, markedly increases when the phosphatidylinositol content is raised from 10 to 30% of the total phospholipids, i.e. in a range found in natural membranes. Addition of gentamicin decreases the activity of these enzymes in a non-competitive fashion, but the effect is inversely proportional to the amount of phosphatidylinositol present in the bilayer. Gentamicin and bis(beta-diethylaminoethylether)hexestrol (DEH), a cationic amphiphile which also binds to phospholipid bilayers, are equipotent inhibitors when added to negatively-charged liposomes at equinormal concentrations. Although direct aminoglycoside-enzyme interactions cannot be excluded, these results strongly suggest that gentamicin impairs the activities of the lysosomal phospholipases towards phosphatidylcholine by decreasing the available negative charges required for optimal activity.