Homogeneous immunoassay of antibody by use of liposomes made of a model lipid of archaebacteria.

Liposomes made of 1,2-di(3RS,7R,11R-phytanyl)-sn-glycero-3-phosphocholine (DPhyPC), which was synthesized as one of the model lipids existing in archaebacterial halophiles, showed excellent stability. Because of this high stability, DPhyPC liposomes could be constituted high ratios (50%) of N-[4-(p-maleimidophenyl) butyryl] dipalmitoyl phosphatidylethanolamine (MPB-DPPE), and consequently could bind large amounts of antigen (alpha-chymotrypsinogen A) on the liposome surface in comparison with those made of ordinary lipids, such as dipalmitoylphosphatidylcholine (DPPC). Though the characteristics of the DPhyPC liposomal membranes in lysis by the classical complement pathway were similar to those of DPPC liposomes, a high sensitivity and a low detection limit in the liposome immune lysis assay (LILA) of antibodies were attained by binding large amounts of the antigen. Further, by coupling sufficient amounts of antigen, almost all the DPhyPC liposome surface was covered with the antigen, and such liposomes showed higher resistance against non-specific lysis caused by complement activity in serum samples, which may be effective in reducing positive-false errors in LILA.