Prevention of nonspecific lysis in liposomal and erythrocyte immunoassay systems by small lipid vesicles and erythrocyte ghosts

Liposomes and immunoassays

Various aspects of the application of liposomes as a label in immunoassays are reviewed. Methods for the preparation of liposomes, from the basic film method to the more advanced dehydration-rehydration method, are discussed. Furthermore, the markers used in liposome labels, as well as the methods to conjugate liposomes to antigens or antibodies, are summarized. Liposome immunoassays are applied as homogeneous or heterogeneous assays. Homogeneous assays often rely on the lytic activity of complement on antibody-associated liposomes. Another group of homogeneous assays utilizes the inhibitory action of antibodies on the activity of conjugates of mellitin (a bee venom protein) with a hapten. Free mellitin conjugates are able to lyse liposomes effectively. Heterogeneous liposome immunoassays, performed either competitively or non-competitively, resemble more closely standard enzyme linked immunosorbent assays, with the enzyme being replaced by a liposome label. Washing steps are used to separate antigen-specifically bound liposomes from unbound liposomes. All bound liposomes are lysed with a detergent, giving an instantaneous amplification. Flow-injection liposome immunoassays and liposome immunosensors are also described as examples of other possible immunoassay formats.

Increased erythrocyte and protein binding of codeine in patients with sickle cell disease

Erythrocyte binding and plasma protein binding of codeine in sickle cell patients and healthy controls were determined. A reversed-phase HPLC procedure was used for codeine analysis. Codeine was extracted from alkalinized plasma, separated on a CN column, and assayed by fluorescence detection. The erythrocyte-buffer partition coefficient was significantly higher in sickle cell patients (1.72 +/- 0.21) than in healthy controls (1.25 +/- 0.14). No time dependence of partitioning was observed. The fraction of codeine bound to plasma proteins, determined by ultrafiltration, was significantly higher in sickle cell patients (66.0% +/- 8.6%) than in healthy controls (30.5% +/- 2.7%). No concentration dependence of erythrocyte or protein binding was observed. Further studies were performed to elucidate the binding mechanisms. From a ghost cell binding study it was concluded that the major binding sites for codeine are in the cell membrane. A decrease in codeine binding was observed in the presence of bilirubin. Codeine binding to alpha 1-acid glycoprotein was found to be minimal. The levels of alpha 1-acid glycoprotein and other glycoproteins in sickle cell patients and healthy controls were measured by glycoprotein electrophoresis. The results showed no significant difference between the two groups. Plasma protein electrophoresis was performed for the two groups. The results showed a significant difference in gamma-globulin levels between sickle cell patients and healthy controls. Codeine is known to bind to gamma-globulin, a fact that may explain in part the observed increase in the plasma protein binding of codeine in sickle cell patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Liposome immune lysis assay (LILA) for gelonin

A complement-mediated liposome immune lysis assay using entrapped calcein was developed for a plant toxin gelonin. Gelonin was covalently coupled to DPPE, and then adsorbed on to the surface of liposomes. Such antigen-bearing liposomes when incubated with anti-gelonin antibody in the presence of guinea pig complement undergo lysis. The detection range is from 3 ng to 60 ng. The method was used to monitor isolation of gelonin by affinity chromatography. It was observed that a minor peak in addition to the major one comes with gelonin, shared common epitopes/epitope with gelonin in immunological reaction. This was further confirmed by SDS-gel electrophoresis indicating the former being an isoform of gelonin. A comparative study of the immunocross-reactivity of ricin and ricin A chain with anti-gelonin antibody was carried out. It was found that while ricin A chain cross-reacted extensively with gelonin antibody and intact ricin elicited little or no cross-reactivity. It is suggested that the present LILA may be employed for the detection and quantitation of ricin A chain by this LILA method.