On the mechanism of the lipoprotein lipase-induced fluorescence changes in dansyl phosphatidylethanolamine-labeled very low density lipoproteins.

Dansyl phosphatidylethanolamine (DPE) (dansyl, 5dimethylaminonaphthalene-1-sulfonyl) was incorporated into human plasma very low density lipoproteins (VLDL). Concomitant with its lipolysis, DPE-VLDL undergoes a greater than %fold fluorescence enhancement, a large increase in fluorescence polarization, and a 20 nm blue shifl in the wavelength of its emission maxima. Based on fluorescence quenching studies with NaI and the measured X, , of DPE and of DPE incorporated into VLDL and phospholipid vesicles, we suggest that DPE is present in the phospholipid monolayer of the VLDL particle. Ultracentrifugation and Sepharose 4B column chromatography of DPE-VLDL showed that the fluorophore is tightly associated with the VLDL particle and is not removed by incubation with albumin. With lipolysis, however, the fluorophore is removed from VLDL and becomes associated with albumin. The lipoprotein lipase-induced fluorescence changes in DPE-VLDL cannot be explained by DPE binding to albumin. Thin layer chromatography revealed that lipoprotein lipase hydrolyzes DPE to produce lyso-DPE. The fluorescence changes that occur with the hydrolysis of DPE-VLDL by lipoprotein lipase may be explained by a mechanism involving the hydrolysis of DPE to lyso-DPE and the subsequent binding of lyso-DPE to albumin. Fluorescence energy transfer was observed from the tryptophyl residues of albumin to lyso-DPE, providing further evidence for their complex formation. The increases in fluorescence intensity, fluorescence polarization, and fluorescence energy transfer that occur with the hydrolysis of DPEVLDL all provide means of monitoring continuously the lipolysis of human very low density lipoproteins by lipoprotein lipase.