Steady-state and time-resolved spectroscopic studies on the hematoporphyrin-lipoprotein complex

The interaction of hematoporphyrin (Hp) with the isolated rabbit lipoprotein fractions very low density lipoproteins, low-density lipoproteins, and high-density lipoproteins has been studied by steady-state and time-resolved spectroscopy. The porphyrin appears to be bound to both the apoprotein and the lipid phase. The two populations of lipoprotein-bound Hp molecules can be distinguished on the basis of the fluorescence excitation spectrum, decay constants of the lowest excited singlet and triplet states, and accessibility to oxygen. Upon Hp binding, the intrinsic fluorescence emission of apolipoproteins is quenched at least in part via singlet-singlet energy transfer from tryptophyl residues to the porphyrin moiety. The binding of Hp with the protein matrix can be adequately described on the basis of Scatchard analysis, whereas the interaction of Hp with the lipid core can be described as the partitioning of the dye between a hydrophobic and an aqueous phase. The Hp binding capacity of lipoproteins is maximal for very low density lipoproteins.