Photometric and electrochemical enzyme-multiplied assay techniques using beta-galactosidase as reporter enzyme

Beta-galactosidase (beta-gal) is shown to be a versatile new reporter enzyme in both photometric and electrochemical enzyme-multiplied assay techniques (EMATs). The well-known beta-gal substrate analog, o-nitrophenyl beta-d-galactopyranoside, yields the visibly colored, o-nitrophenol product upon hydrolysis, whereas the substrate, p-aminophenyl beta-D-galactopyranoside, gives rise to an electrooxidizable product, p-aminophenol. These beta-gal substrates made possible the demonstration of both photometric and electrochemical signal transduction schemes for beta-gal-based EMAT detection of estradiol (as the estradiol-bovine serum albumin (E-BSA) conjugate). The EMAT system is composed of the reporter enzyme, beta-gal, with covalently attached estradiol, and estrogen antibody, which inhibits enzyme activity of the beta-gal-estradiol conjugate up to approximately 75%. Reporter enzyme inhibition is relieved significantly by addition of < or =2 ng/mL of estradiol (as E-BSA), which competes for binding with the antibody. Thus, the presence of analyte (E-BSA) is reported by the enzyme (beta-gal), which amplifies the ligand-protein dissociation event by turning over its substrate repeatedly. The electrochemical version of EMAT, based on amperometric detection of p-aminophenol, is responsive to added estradiol within minutes. These results show that beta-gal may serve as a useful alternative to glucose-6-phosphate dehydrogenase, which currently is used as reporter enzyme in commercially available EMAT systems.