Analytical applications of liposomes
Liposomes have established roles in drug delivery and
cell membrane studies. Amongst other applications; they
can also be used as analytical reagents, particularly in
immunoassays. Liposomal immunoassays have potential
advantages over alternatives; including sensitivity,
speed, simplicity and relative reagent stability. The aim
of these studies was to develop and evaluate novel
examples of these assays.
When liposomes entrapped the dye, Sulphorhodamine B, a
shift in its maximum absorption wavelength compared to
free dye was observed. This was attributed to
dimerization of the dye at high concentrations. If the
liposomes were disrupted, the released dye was diluted
into the external buffer, and the dye's absorption
spectrum reverted to that of free dye.
After optimization of dye entrapment, immunoassays were
developed using these liposomes. Albumin-coated
liposomes were used in a model assay to measure serum
albumin. This assay employed complement-mediated
immunolysis, commonly used in liposomal immunoassays.
The liposomes were lysed by anti-albumin and complement,
and this could be competitively inhibited by serum albumin.
To improve sensitivity, Fab' anti-albumin liposomes were
prepared. These enabled measurement of urinary albumin
by a complement-mediated immunoassay, but using a
Anti-albumin (intact) liposomes were shown to precipitate
on gentle centrifugation after reaction with albumin.
They were applied as a solid phase reagent in an
heterogeneous immunoassay, using radioimmunoassay for
urinary microalbumin as a model assay.
Liposomes containing Sulphorhodamine B were also used in
a more novel assay; for serum anticardiolipin antibodies.
Cardiolipin-containing liposomes were prepared. These
were lysable using magnesium ions. Anticardiolipin
antibodies (IgG) were found to augment this lysis,
enabling their estimation. Similar imprecision and
acceptable correlation with a commercial enzyme-linked
immunosorbent assay (ELISA) were obtained.
The findings demonstrate Sulphorhodamine B release can be
used as a marker in homogeneous colorimetric liposomal
immunoassays; both in model assays and in potentially
more useful clinical biochemistry applications.