An Enzyme Cascade-Triggered Fluorogenic and Chromogenic Reaction Applied in Enzyme Activity Assay and Immunoassay.

An enzyme cascade-triggered reaction with novel signal generation mechanism is beneficial for the development and insight of the enzyme cascade, which is extensively used for signal transduction in potential applications. Inspired by the fluorogenic and chromogenic reaction between dopamine and resorcinol, and the specific catalytic properties of alkaline phosphatase (ALP) and tyrosinase, we designed and synthesized an unconventional substrate of ALP, named p-aminoethyl-phenyl phosphate disodium salt (PAPP). As expected, the ALP and tyrosinase-incubated PAPP solution exhibited pale yellow with intense blue fluorescence upon addition of resorcinol, owing to the ALP-catalyzed transformation of PAPP into an intermediate tyramine, and the tyrosinase-catalyzed hydroxylation of tyramine to dopamine, as well as the specific reaction between dopamine and resorcinol. Therefore, an enzyme cascade system has been developed herein based on the ALP and tyrosinase coupled enzymes-triggered fluorogenic and chromogenic reaction. According to the direct relationship between the activity of ALP/tyrosinase and absorbance/fluorescence intensity of the resultant solution, the proposed enzyme cascade-triggered reaction was utilized for assaying ALP and tyrosinase activity with fluorometric and colorimetric dual-readout signals. Furthermore, such enzyme cascade catalysis process was integrated into the ALP-based cascade enzyme-linked immunosorbent assay with dual-readout signals, resulting in the sensitive detection of cardiac troponin I in diluted serum.