Lipid thermotropic transitions in Triatoma infestans lipophorin

The structure and lipid thermotropic transitions of highly purified lipophorin of Triatoma infestans were examined by several techniques: steady-state fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH), cis-parinaric acid (cis-PnA) and trans-parinaric acid (trans-PnA), light scattering fluorescence energy transfer between the lipophorin tryptophan residues and the bound chromophores, DPH, trans-parinaric acid cis-parinaric acid, gel electrophoresis, and gel filtration. Fluorescence polarization of PnAs and DPH revealed a reversible lipid thermotropic transition in intact lipophorin at about 20 degrees C and 18 degrees C, respectively. In lipophorin, lipid dispersion fluorescence polarization of DPH detected a lipid transition approximately at 20 degrees C, while trans-PnA showed a gel phase formation at a temperature below 30 degrees C. Similar experiments in which trans-PnA was incorporated into diacylglycerols and phospholipids extracted from the lipophorin revealed gel phase formation below 30 degrees C and 24 degrees C, respectively. Light scattering measurements showed that lipophorin particles aggregate irreversibly at 45 degrees C, increasing the molecular weight, as determined by gel filtration on Sephacryl S-300, from 740,000 to values larger than 1,500,000. The particle aggregation did not change the physical properties of the lipophorin studied by fluorescence polarization, indicating that the aggregation is apparently a non-denaturing process. Energy transfer between the lipophorin tryptophans and the bound chromophores cis-PnA, trans-PnA, and DPA revealed a different location of the fluorescent probes within the lipophorin. Temperature-dependence on the energy transfer efficiency for all probes confirmed a change in the ordering of the lipophorin lipids at 24 degrees C.