Kinetic analysis of the interaction between liposomes and the complement system in rat serum: re-evaluation of size-dependency.

The size of liposomes is considered to be an important factor in determining the liposome-complement interaction. In this study, the release of carboxyfluorescein (CF) from liposomes was measured continuously for three different diameters (800, 400 and 200 nm) by changing the liposome concentration from 1 to 1000 nmol/ml. At a low liposome concentration range (1-10 nmol/ml), small liposomes (200 nm) released CF to a similar extent (approximately 35%) as in the medium (400 nm) and large (800 nm) liposomes. The affinity (Km) and capacity (Lmax) of a complement system to release liposomally encapsulated CF were estimated by kinetic analysis of the liposome-complement interaction. Surprisingly, there was no remarkable size dependency in the Km and Lmax in terms of liposome number, although these parameters depended on the size of liposomes in terms of lipid concentration. These results indicated the possibility that the complement system does not discriminate according to liposome size.