A variety of chromogenic substrates to peroxidase have been reported in the literature as being useful analytical techniques for the detection and measurement of peroxidase activity. Analytical techniques utilizing peroxidase or peroxidase coupled to antibodies and other molecules serve in the measurement of biological properties and components of compounds of interest. Typical components include among others glucose, maltose, bacteria, viruses, enzymes, drugs, hormones and the like. for example, it is known that chromogenic substrates such as benzidine, 0-tolidine, 0-toluidine, and compounds such as 3-methyl-2-benzothiazolinone hydrazone and 3-(dimethylamino)benzoic acid can be used to monitor peroxidase activity. However, certain of these compounds have been shown to be carcinogenic and very sensitive to light, oxygen from the air and hydrogen peroxide.
In an article by T. T. Ngo and H. M. Lenhoff, Analytical Biochemistry, 105, 389-397 (1980), a chromogenic assay for peroxidase and peroxidase-coupled reactions is disclosed and it is indicated that the assay is sensitive and versatile. This assay is based on the oxidative coupling of 3-methyl-2-benzothiazolinone hydrazone and 3-(dimethylamino)benzoic acid. In the presence of hydrogen peroxide and the aforementioned two compounds, peroxidase catalyzes the formation of a deep purple compound, most likely an indamine dye, which has a broad absorption band between 575 and 600 nm with a peak at 590 nm. Using this assay system solutions of peroxidase can be determined in picomolar amounts by either a rate or a fixed-time method.
In the Journal of Immunological Methods, 60, 61-68 (1983) W.D. Geoghegan et al reported the use of the above Ngo-Lenhoff peroxidase assay for solid phase ELISA. While acknowledging the extreme sensitivity of the method and the rapid production of the indamine dye from the chromogen, the authors indicated that problems were encountered in the adaption of the assay to ELISA. for example, the blank developed color, exposure to light resulted in increased dye production, and other problems. However, by the use of various buffer systems, citric acid, and the like, it was possible for the authors to utilize the Ngo-Lenhoff assay for solid phase ELISA.
Notwithstanding the sensitivity and versatility of the Ngo-Lenhoff method, a chromogenic substrate would be desirable which could exhibit excellent stability against ultraviolet light, and which would not be affected by either hydrogen peroxide or peroxidase alone. It was observed that the presence of hydrogen peroxide alone was detrimental to the Ngo-Lenhoff assay. Additionally, known immunodiagnostic systems usually require that the reaction be terminated by the addition of acid or other chemicals.
Accordingly, one or more of the following objects will be achieved by the practice of the present invention. It is an object of this invention to provide a chromgenic substrate which is useful in the quantitative and qualitative determination of peroxidase in biological substances, particularly biological fluids. A still further object of the present invention is to provide a unique chromogenic substrate to peroxidase which forms upon oxidative interaction a dye which can easily be observed by the naked eye, or determined qualitatively or quantitatively by absorption. Another object of this invention is to provide a substrate to peroxidase which exhibits superior stability against untraviolet light, oxygen of the air, and hydrogen peroxide. A still further object of this invention is to provide a method for detecting peroxidase activity in biological samples or immunodiagnostic formats involving peroxidases by employing the substrate of the present invention. Another object is to provide substrates which can be used in immunodiagnostic systems without the need to terminate the reaction by the addition of acids or other chemicals. These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein set forth.