Pesticide Biochemistry and Physiology 70, 74–85 (2001) doi:10.1006/pest.2001.2539, available online at http://www.idealibrary.com on The Organophosphorus Insecticide Parathion Changes Properties of Natural and Model Membranes Mario Suwalsky,* ,1 Patricio Ramos,* Fernando Villena,² Herna´n Ca ´rdenas,² Beryl Norris,² Francisco Cuevas,‡ and Carlos P. Sotomayor‡ *Faculty of Chemical Sciences and ² Faculty of Biological Sciences, University of Concepcio ´n, Casilla 160-C, Concepcio ´n, Chile; and ‡Institute of Chemistry, Catholic University of Valparaı ´so, Valparaı ´so, Chile Received November 8, 2000; accepted January 29, 2001 The interaction of parathion, a widely used organophosphorus insecticide, with cell membranes of the isolated toad skin, human erythrocytes, and membrane molecular models is described. The latter consisted of multilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE) and large unilamellar vesicles (LUV) of DMPC. They represent phospholipid classes located in the outer and inner monolayers of the human erythrocyte membrane, respectively. Results obtained from electrophysiological measurements of the toad skin supported the deduction that the main site with which parathion interacted to reduce ion transport is situated on the outward facing barrier of the epithelium. Fluorescence spectroscopy showed that the insecticide interacted with and altered the structure of isolated unsealed human erythrocyte membranes. The increase of the fluorophor DPH anisotropy and of laurdan general polarization indicated an ordering effect on the acyl chains and a decrease in the molecular dynamics and/or water penetration at the lipid polar head group region. However, in DMPC LUV parathion at increasing concentrations enhanced the polar head group ordering but decreased that of their acyl chains. X-ray analysis of DMPC and DMPE multilayers showed that both structures were deeply perturbed by a parathion:lipid ratio as low as 1:200. Nonetheless, increasing parathion concentrations induced different results in both lipid bilayers.  2001 Academic Press Key Words: parathion; insecticide; cell membrane; erythrocyte membrane; phospholipid bilayer. INTRODUCTION persistent and are manufactured inexpensively. Due to their lipophilic character they can be The widespread use of pesticides in an attempt potentially hazardous because of their transport to increase crop production has generated a through the food chain and accumulation in the series of toxicological and environmental prob- lipid phase of cellular and subcellular mem- lems, in particular the appearance of resistant branes. With the restriction on the use of most species of pests, toxic effects in man and ani- organochlorine pesticides imposed in the 1970s, mals, and the persistence of pesticides in the the less persistent but highly effective organo- environment. In particular, the use of organo- phosphorus agents became the insecticides of chlorine pesticides has created the greatest con- choice. Parathion, whose structural formula is cern as they are designed precisely to be shown in Fig. 1, is a widely used insecticide, included in the 38 different organophosphorus 1 To whom correspondence should be addressed. Fax: 56- pesticides registered and approved for use in the 41-245974. E-mail: msuwalsk@udec.cl. United States (1). Unfortunately, these com- 2 Abbreviations used: IUM, isolated unsealed human pounds may exhibit mammalian toxicity and can erythrocyte membrane; DMPC, dimyristoylphosphatidyl- choline; DMPE, dimyristoylphosphatidylethanolamine; be unselective with respect to target species. DPH, 1,6-diphenyl-1,3,5-hexatriene; laurdan, 6-dodecanoyl- The acute toxicity of most of these pesticides is 2-dimethylaminonaphthalene; LUV, large unilamellar vesi- initiated by inhibition of the enzyme acetylcho- cles; r, fluorescence anisotropy; GP, general polarization; linesterase in the central and peripheral nervous PD, potential difference; Isc, short-circuit current; SE, stan- dard error. systems in which the active site of the enzyme, 74 0048-3575/01 $35.00 Copyright  2001 by Academic Press All rights of reproduction in any form reserved.