Sensitive detection of alkaline phosphatase based on terminal deoxynucleotidyl transferase and endonuclease IV-assisted exponential signal amplification

Alkaline phosphatase (ALP) is widely expressed in human tissues. ALP plays an important role in the dephosphorylation of proteins and nucleic acids. Therefore, quantitative analysis of ALP plays a vital role in disease diagnosis and the development of biological detection methods. Terminal deoxynucleotidyl transferase (TdT) catalyzes continuous polymerization of deoxynucleotide triphosphates at the 3'-OH end of single-stranded DNA in the absence of a template. In this study, we developed a highly sensitive and selective method based on TdT and endonuclease IV (Endo IV) to quantify ALP activity. After ALP hydrolyzes the 3'-PO₄ end of the substrate and generates 3'-OH, TdT can effectively elongate the 3'-OH end with deoxynucleotide adenine triphosphate (dATP) and produce a poly A tail, which can be detected by the poly T probes. Endo IV digests the AP site in poly T probes to generate a fluorescent signal and a new 3'-OH end, leading to the generation of exponential fluorescence signal amplification. The substrate for TdT elongation was optimized, and a limit of detection of 4.3×10^-3 U/L was achieved for ALP by the optimized substrate structure. This method can also detect ALP in the cell lysate of a single cell. This work has potential applications in disease diagnosis and biomedical detection.