DETACHMENT OF CYTOCHROME-C BY CATIONIC DRUGS FROM MEMBRANES CONTAINING ACIDIC PHOSPHOLIPIDS - COMPARISON OF LIDOCAINE, PROPRANOLOL, AND GENTAMICIN

A large number of pharmaceutically active compounds have a high affini ty to acidic phospholipids; good examples are the cationic compounds l idocaine, propranolol, and gentamycin. These drugs influenced the lipi d dynamics of liposomes composed of phosphatidylcholine and the acidic phosphatidylglycerol, as judged by the excimer/monomer emission inten sity ratio for a pyrene-labeled phospholipid analog, as well as by pol arization of DPH fluorescence. When the mole fraction X of PG (X-PG) w as 0.20, lidocaine increased membrane fluidity. The opposite was true for propranolol, which caused the formation of pyrene lipid-enriched m icrodomains. Gentamycin had no apparent effect. At X-PG = 1.00, all th ese drugs rigidified membrane. Subsequently, we Investigated the detac hment of a cationic peripheral membrane protein, cytochrome c (cyt c), by these compounds from liposomes. This was accomplished by monitorin g resonance energy transfer from a pyrene-labeled phospholipid to the heme of cyt c. The efficiency of the above compounds to dissociate cyt c varied considerably. In brief, significantly lower concentrations o f gentamycin than propranolol or lidocaine were required for half-maxi mal dissociation of cyt c from liposomes, although the final extent of protein detachment by gentamycin was less complete. ATP augmented the dissociation of cyt c from membranes by lidocaine and propranolol. St opped-flow measurements also revealed that the half-times differed for the release of cyt c from the membranes. Our results are likely to re flect differences in the contributions of the electrostatic interactio ns and hydrophobicity to the drug/lipid interaction and comply with tw o different acidic phospholipid binding sites in cyt c.