3-p-Hydroxyphenylpropionic Acid—A Sensitive Fluorogenic Substrate for Automated Fluorometric Enzyme Immunoassays

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Abstract
The application of 3-p-hydroxyphenylpropionic acid (HPPA), a fluorogenic substrate of horseradish peroxidase (HRP) to an automated microplate fluorometric enzyme immunoassay is described. Fluorescence intensity of the end product was highly dependent on the pH of the buffer and on the concentrations of the substrate mixture ingredients. The distinct advantage of a fluorometric assay is the possibility to perform a quantitative detection of analyte over a very wide dynamic range. Clinical evaluation of both assays showed good correlation between the FEIA and conventional methods. The determination of human thyrotropin (TSH) and recombinant hepatitis B surface antigen (rHBsAg) were performed using a fluorometric enzyme immunoassay (FEIA) with HPPA as the substrate, and a colorimetric one with tetramethylbenzidine (TMB) as the chromogenic substrate. The sensitivity of both types of assays proved comparable.