Spin membrane immunoassay: simplicity and specificity.

The many disadvantages of radioimmunoassay (RIA) have stimulated attempts to develop non-radioactive assays. These include spin immunoassay (SIA), which is simple, specific, and requires no separation procedures but is much less sensitive than RIA. The membrane immunoassay (MIA) described here is more sensitive than the SIA. Serum is prepared as for RIA. The MIA employs liposomes sensitized with epsilon-dinitrophenylated aminocaproyl phosphatidylethanolamine. It records liposome lysis induced by specific anti-Dnp antibodies, and complement which is monitored by the release of trapped spin labels (N-(2,2,6,6-tetramethylpiperidinyl-1-oxyl)-choline chloride). The sensitized liposomes are stable and give reproducible results for up to 4 weeks. The system's sensitivity is limited by the antibody's affinity (Ka approximately 10(8) M-1) rather than the sensitivity of the electron spin resonance spectrometer (approximately 1 X 10(-7) M). The inhibition of liposome lysis is hapten specific: (epsilon-Dnp-aminocaproic acid,epsilon-Dnp-lysine) greater than alpha-Dnp-glycine; o-nitroaniline and epsilon-dansyl-lysine are ineffective. Inhibition is quantitative without augmentation.