Fluorescent lipoprotein probe

The fluorescent cholesterol analog cholesta-5,7,9(11)triene-3-β-ol was used to label high-density and low-density lipoproteins in vivo (rabbit) and in vitro (human). Rabbit feeding experiments demonstrated that in vivo both the esterified and nonesterified forms of the fluorophore were incorporated by these particles. Using in vitro labeling techniques, it was possible to selectively incorporate either the free form of the fluorophore or both the free and the esterified forms depending upon the presence or absence of serum esterases during incubation. Subsequent to labeling, the thermotropic behavior of the low- and high-density lipoproteins was evaluated using temperature-dependent fluorescence intensity measurements. Purified low-density lipoprotein samples (human and rabbit) containing both forms of the fluorophore were observed to undergo a thermotropic transition between 27 and 32°C. However, this transition was not observed in low-density lipoprotein samples containing only the nonesterified form of the probe nor was it observed in any of the high-density lipoprotein samples, even those containing both forms of the fluorophore. These results provide further evidence that the previously reported thermotropic transition in low-density lipoprotein is due to a reordering of the low-density lipoprotein cholesterol ester core (Deckelbaum, R.J., Shipley, G. G., Small, D. M., Lees, R. S., and George, P. K. (1975) Science190, 392–394; Sears, B., Deckelbaum, R. J., Janiak, M. J., Shipley, G. G., and Small, D. M. (1976) Biochemistry15, 4151–4157; Chana, G. S., Sheppard, R. J., Mills, G. L., and Grant, E. H. (1980) Phys. Med. Biol.25, 427–432).