Homogeneous substrate-labeled fluorescent immunoassay for human serum albumin

A homogeneous substrate-labeled fluorescent immunoassay for human serum albumin (HSA) has been developed, similar to previously described immunoassays for Immunoglobulin G and Immunoglobulin M. HSA was covalently linked to 6-(7-beta-galactosylcoumarin-3-carboxamide) hexylamine. The resulting conjugate had minimal fluorescence at 450 nm (with excitation at 400 nm). However, when the acetal linkage of the galactosyl moiety was hydrolyzed by beta-galactosidase, a substantial increase in the fluorescence was obtained. This increase was specifically inhibited by antibody to HSA. A competitive binding immunoassay was established by letting the conjugate compete with HSA in the serum for the limited number of antibody-binding sites. The level of fluorescence resulting from the addition of enzyme was proportional to the amount of HSA in the serum. Precision, analytical recovery and serum dilution studies were carried out on the assay. The immunoassay was compared to an albumin assay using the dye-binding method.