COMPETITIVE IMMUNOSORBENT ASSAYS FOR BIOTIN USING BIFUNCTIONAL UNILAMELLAR VESICLES

Competitive immunosorbent assays for the model antigen biotin were per formed using both unilamellar vesicles with covalently attached biotin and horseradish peroxidase (HBVs) and commercially available biotin-l abeled horseradish peroxidase (B-HRP) as the enzyme-labeled antigen. T he assays were performed using anti-biotin antibody (ABA) surface dens ities ranging from one-tenth to full monolayer coverage. It was found that assays using HBVs strongly depended on the antibody surface densi ty, while assays using B-HRP were relatively insensitive to the antibo dy surface density. The HBV assay dependence on the ABA surface densit y was most likely due to multiple point attachment of vesicles to the surface. The lowest detectable antigen concentration (least detectable dose) for vesicles (similar to 10(-9) M) was an order of magnitude lo wer than the value found for B-HRP (similar to 10(-8) M). The sensitiv ity (slope of the response vs biotin concentration curve) of assays wi th B-HRP was comparable to the sensitivity of assays with HBVs at low antibody surface density, probably due to less extensive multipoint at tachment. It was also found that assays could be performed with vesicl es at antibody surface densities that were at least 5 times lower, in terms of the bulk antibody concentrations used to coat the wells, than antibody surface densities at which B-HRP gave comparable signals (si milar to 1 Delta A/min).