STRUCTURAL STUDIES ON THE INTERACTION OF DRUGS WITH CELL MEMBRANES

Mario Suwalsky¹, Pedro Hernández, Fernando Villena and Carlos P. Sotomayor²

¹University of Concepción, Casilla 3-C, Concepción, Chile and
²Catholic University of Valparaíso, Valparaíso, Chile.

Keywords: Drugs, cell membrane, phospholipid bilayers.

Very little is known about the three-dimensional structure of most cell membranes. This is due to two related reasons. The first is that although proteins and lipids constitute about one half of their dry weight, the number of different molecules is quite high. The second is that these molecules show a very low degree of periodical order, aggravated by the fluidizing action of water. Membranes are, therefore, very complex systems. This has led to the proposal of several different models of which that of Singer and Nicolson has been widely accepted. In essence, the lipids would arrange themselves in asymmetric bilayers in which the proteins are irregularly inserted. Given the complexity of cell membranes, simpler models based on phospholipid bilayers are widely used. In our X-ray lab we have determined the structure of phospholipids located in the outer and inner monolayers of cell membranes. Besides, we have studied the perturbing effect of water upon them (1). Since then we have been using phospholipid bilayers as molecular models to study the way different chemicals of biological interest such as therapeutical drugs (2-3), pesticides (4-5) and contaminant cations (6) interact with cell membranes. On this occasion we will present the results obtained with antiarrhythmic and anticancer drugs.

The antiarrhythmic drugs (AAD) that belong to class I exert their action in the myocard cell membrane by blocking the sodium channels. Five possible molecular mechanisms have been suggested of which three involve nonspecific interactions with the membrane phospholipids; as a result, the channel functions would be altered. To probe the validity of these hypotheses three AAD with different degrees of lipophilicity were made to interact with a) bilayers of dimyristoylphosphatidyl-ethanolamine (DMPE) and of dimyristoylphosphatidylcholine (DMPC), representative of phospholipid classes located in the inner and outer monolayers of most cell membranes, respectively. The interactions were performed in both a hydrophobic and aqueous media, which were studied by X-ray diffraction, b) DMPC vesicles, studied by fluorescence spectroscopy, and c) human erythrocytes, which were observed by scanning electron microscopy (SEM). It was found that the three AAD interacted with both phospholipid bilayers. However, the extension of the interactions depended on the nature and concentration of the lipids and the drugs as well as on the medium where the interactions were performed. On the other hand, the observations by SEM showed that the three AAD produced profound shape alterations to the incubated erythrocytes. However, the type and intensity of these changes were dependent on the lipophilicity of the drugs and their concentrations. It was concluded that the AAD might affect the Na channel functions through their interactions with neighboring phospholipid bilayers.

Most of the anticancer drugs (ACD) are designed to interact with DNA or to interfere with the synthesis of nucleic acids. However, in the last decade the attention has been centered on the cell membranes. In fact, many of them interact and affect their properties and functions such as the ionic transport and fluidity, as well as on some of their proteins, particularly of receptors. The interactions of two ACD, tamoxifen and adriamycin, with cell membranes were studied in the same way as described above. The observed results indicated that these drugs indeed interacted with cell membranes.

Acknowledgments. Research grants from FONDECYT (1960680) and DIUC (95.24.09-1) are acknowledged.

1.- M. Suwalsky, "Structural Studies on Phospholipid Bilayers by X-ray Diffraction Methods, in Physical Properties of Biological Membranes and their Functional Implications" (C. Hurtado, ed.), Plenum, chapter 1, 1988
2.- M. Suwalsky, I. Sánchez, M. Bagnara and C.P. Sotomayor, Interaction of Antiarrhythmic Drugs with Model Membranes, Biochim. Biophys. Acta 1195 (1994) 189-196
3.- M. Suwalsky, F. Villena, F. Aguilar and C.P. Sotomayor, Interaction of Penicillin G with the Human Erythrocyte Membrane and Models, Z. Naturforsch. 51c (1996) 243-248
4.- M. Suwalsky, M. Benites, F. Villena, F. Aguilar and C.P. Sotomayor, The Organochlorine Pesticide Heptachlor Disrupts the Structure of Model and Cell Membranes, Biochim. Biophys. Acta 1326 (1997) 115-123
5.- M. Suwalsky, M Benites, F. Villena, F. Aguilar and C.P. Sotomayor, Interaction of the Organochlorine Pesticide Dieldrin with Phospholipid Bilayers, Z. Naturforsch. 52c (1997) 450-458
6.- M. Suwalsky, B. Ungerer, L. Quevedo, F. Aguilar and C.P. Sotomayor, Cu²⁺ Ions Interact with Cell Membranes, J. Inorg. Biochem. (in press).

Corresponding address: Faculty of Chemical Sciences, University of Concepción, Chile;

Fax : 56-41-245974, Telephone: 56-41-204171, e-mail: msuwalsk@udec.cl