Interaction of alpha-lactalbumin with dimyristoylphosphatidylcholine vesicles. III. Influence of the temperature and of the lipid-to-protein molar ratio on the complex formation.

We investigated the interaction between alpha-lactalbumin and sonicated dimyristoylphosphatidylcholine at pH 4 and different temperatures. (1) At 23 degrees C and lipid-to-protein molar ratios below 170, the interaction results in a disruption of the original vesicles to form smaller complex particles. By the sedimentation velocity method we determined for this particle a molar mass of (1.05 +/- 0.16) X 10(6) g X mol-1. The lipid-to-protein molar ratio within the complex particle is 70/1, as earlier estimated. It follows that there are approximately 1200 lipid and 17 alpha-lactalbumin molecules per particle. At molar ratios above 170, alpha-lactalbumin strongly associates with the vesicles. In this case the vesicle entity remains. The ability of alpha-lactalbumin to break up the vesicles at this temperature is determined by the number of protein molecules which are required in the complex particle. (2) By means of fluorescence polarization of the lipophilic probe 1,6-diphenyl-1,3,5-hexatriene and energy transfer of the tryptophan groups of the protein to 1,3-(1,1'-dipyrenyl)propane located in the hydrocarbon region of the vesicles, it is shown that with increasing temperature above 25 degrees C, complexes of decreasing internal lipid-to-protein molar ratio are formed. However, by electron microscopy we show that the overall size of these complexes remains approximately the same, i.e., bars with dimensions 70 X 220 A. A temperature-reversible transformation occurs between these complexes, which cannot be isolated by gel chromatography. In contrast, the complex of molar ratio 70/1 remains stable at lower temperatures.

An intramolecular excimer forming probe used to study the interaction of alpha-lactalbumin with model membranes.

The nonconjugated bichromophoric molecule 1,3-di(1-pyrenyl)propane shows, besides the pyrene monomer fluorescence, a structureless emission due to an intramolecular excited dimer (excimer). In the case of intramolecular excimer forming systems, the ratio of the emission intensities of excimer vs. monomer (IE/IM) is sensitive to changes in membrane structure as will be illustrated here. The present molecule is a useful probe to report on lipid-protein interactions, at least in our model system. We have introduced it into the hydrocarbon layer of dimyristoylphosphatidylcholine vesicles in order to study their interaction with alpha-lactalbumin (alpha-LA) as a function of pH and temperature. On the basis of steady-state fluorescence, kinetic, and energy transfer studies, we have found that, at pH 4, alpha-LA strongly interacts with the lipid bilayer. In steady-state fluorescence experiments, changes of the ratio IE/IM and shifts of the transition temperature have been observed, reflecting changes of membrane structure caused by interaction with alpha-LA. Kinetic studies of the rate of interaction of alpha-lactalbumin and measurements of energy transfer from excited tryptophan(s) to the fluorescent probe confirm the steady-state experiments. Our results agree with previously reported microcalorimetric, gel chromatographic, and fluorescence polarization studies.