DDT - MEMBRANE INTERACTIONS STUDIED WITH 2 FLUORESCENT-PROBES

The effects of DDT (0-100-mu-M) in pure phospholipid and phospholipid/cholesterol bilayers were investigated by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH), a probe located in the bilayer interior, and by excimer to monomer fluorescence intensity ratio (I'/I) of 1,3-di(1-pyrenyl)propane (Py(3)Py), a probe sitting closer to the polar region. In the gel phase of dimyristoylphosphatidylcholine (DMPC) bilayer, DDT induces concentration-dependent fluidizing effects into the hydrophobic membrane regions, but no effects are observed in the outer regions of the membrane, as evaluated by DPH and Py(3)Py, respectively. Furthermore, in the fluid phase of DMPC and for DDT concentrations higher than 10-mu-M, I'/I of Py(3)Py decreases, reflecting an order increase of the probe environment and DPH fails to detect any apparent effect. Similar effects were observed in other pure lipid bilayers, namely dipalmitoylphosphatidylcholine (DPPC) and distearoylphosphatidylcholine (DSPC). Unlike DPH, Py(3)Py is very sensitive to DPPC and DSPC pre-transitions which are not abolished by 50-mu-M DDT. This DDT concentration inhibits to some extent the cholesterol-induced ordering in DMPC bilayers and high cholesterol concentrations (greater-than-or-equal-to 30 mol%) do not prevent insecticide interaction, as evaluated by DPH. On the other hand, the effects of DDT reported by Py(3)Py depend on temperature and cholesterol content of DMPC bilayers. Thus, for cholesterol levels ranging from 10 to 50 mol% and for temperatures below the phase transition temperature of DMPC, Py(3)Py fails to detect any significant effect. Nevertheless, above the phase transition temperature of DMPC, Py(3)Py detects either ordering effects of DDT at low cholesterol contents (< 20 mol%) or fluidizing DDT effects at high cholesterol levels (> 20 mol%).