Cholestatriene and ergostatetraene as in vivo and in vitro membrane and lipoprotein probes.

The fluorescent cholesterol analogues, cholesta-5,7,9(11)triene-3-beta-ol (I) and ergosta-5,7,9(11)-22-tetraene-3-beta-ol (II), have been shown to be readily incorporated by various tissues and lipoproteins in rabbits maintained on diets supplemented with these fluorophores. Human erythrocytes and lipoproteins were also found to incorporate I and II in vitro under physiological conditions. The thermotropic behavior of the lipoproteins and erythrocyte membranes labeled with sterols I and II was evaluated using temperature-dependent fluorescence polarization and/or fluorescence intensity spectra. Erythrocyte ghosts, fluorescently labeled in vivo (rabbit) or in vitro (rabbit and human), were found to undergo a reversible thermally induced transition at 24 +/- 2 degrees C. A similar transition occurring at higher temperatures was also observed in fluorescently labeled human and rabbit LDL particles. Furthermore, the transition temperatures and relative microviscosities of the in vivo labeled rabbit LDL particles were found to be dependent upon the amount of sterol present in the rabbits' diet. No evidence of a similar thermotropic transition was observed in any of the HDL particles. These results are discussed in terms of a thermotropic reordering of cholesterol clusters existing in the erythrocyte membrane and of the cholesteryl ester core present within the low density lipoprotein particle.