Mechanism of cationic amphiphilic drug inhibition of purified lysosomal phospholipase A1.

Cationic amphiphilic drugs like chlorpromazine, propranolol, and chloroquine inhibit lysosomal phospholipase A in vitro. Some workers have proposed that cationic amphiphilic drugs inhibit the activity of phospholipase A1 by forming substrate-drug complexes which cannot be degraded while others have reported competitive inhibition implying drug effects on the enzyme. To analyze the mechanism of inhibition, we examined the binding ability of these drugs to unilamellar vesicles of dioleoylphosphatidylcholine and correlated these results with a detailed kinetic analysis of phospholipase A. Chlorpromazine and propranolol bound to small unilamellar liposomes of dioleoylphosphatidylcholine substrate in a positive cooperative way consistent with two binding sites: a high-affinity site with low capacity and a low-affinity site with high capacity. The affinity of chlorpromazine for the high-affinity site was 2 times greater than that of propranolol (KA = 13 807 +/- 1722 vs. 8481 +/- 1078 M-1), and the saturation number for chlorpromazine was 3 times greater than for propranolol (N = 0.20 +/- 0.004 vs. 0.07 +/- 0.02 mol of drug/mol of phosphatidylcholine). Chloroquine did not bind to unilamellar liposomes of dioleoylphosphatidylcholine. We carried out detailed kinetic studies using purified lysosomal phospholipase A1 from rat liver. In the case of chloroquine inhibition, the Lineweaver-Burk double-reciprocal plots showed straight lines, but the slope replots were curved, indicating the formation of complexes having 2 mol of chloroquine/mol of enzyme (EI2 complexes). Thus, chloroquine is a competitive inhibitor which forms EI2 complexes with phospholipase A1. However, in the case of chlorpromazine and propranolol, the observed kinetic data do not fit to the same equilibrium used for the case of chloroquine.(ABSTRACT TRUNCATED AT 250 WORDS)