In vitro interaction of zinc(II)-phthalocyanine-containing liposomes and plasma lipoproteins

We have studied the interaction of small unilamellar liposomes containing zinc(II)-phthalocyanine (Zn-Pc) with human plasma lipoproteins. High-, low- and very low-density lipoproteins (HDL, LDL and VLDL), were purified from plasma and combined in amounts reflecting their natural abundance in plasma. After short periods of incubation at 37 degrees C, the bulk of Zn-Pc was incorporated into HDL and LDL; very little 14C-labelled palmitoyl oleoyl phosphocholine, the most abundant phospholipid in the formulation, was associated with lipoproteins. When liposomes were incubated in pooled plasma, 73%-85% of Zn-Pc and 27%-34% of radiolabelled phospholipid were recovered with HDL and LDL, indicating a possible role for plasma lipid transfer proteins in the incorporation of phospholipid into lipoproteins. Some Zn-Pc was also found in association with VLDL. The buoyant density of Zn-Pc liposomes increased in a dose-dependent fashion when the particles were incubated with plasma, and it is suggested that this was due, at least in part, to opsonization of liposomes by plasma proteins.