A novel fluorogenic substrate for detecting alkaline phosphatase activity in situ.

We describe here the in situ detection of alkaline phosphatase (APase) activity with a new fluorogenic substrate, 2-(5'-chloro-2'-phosphoryloxyphenyl)-6-chloro-4-(3H)-quinazolinone (CPPCQ). CPPCQ is very soluble and colorless. APase converts it into a rapidly precipitating product, whose strong fluorescence marks the sites of APase activity. The detected APase was either a probing enzyme anchored to epidermal growth factor (EGF) receptors of fixed human epidermoid carcinoma cell line (A431) by biotinylated EGF and streptavidin-APase conjugates or an endogenous marker existing in a fixed canine kidney cell line (MDCK). With CPPCQ staining, the EGF receptors and the endogenous APase were both visualized by fluorescence microscopy as contrasting, photostable, and well-resolved fluorescent stains. The EGF receptor staining was specific since it could be blocked by excessive unlabeled EGF. In contrast, fluorescein-labeled EGF failed to specifically stain the EGF receptors under the same fluorescent microscope. The endogenous APase staining with CPPCQ was sensitive to heating, levamisole and L-homoarginine, showing an APase tissue specificity of the liver/bone/kidney type. Therefore, CPPCQ appears to be a novel substrate dye for sensitive fluorescence APase histochemistry.