Substrates Substrate for the histochemical demonstration of acid and alkaline phosphatase. Application Naphthol AS-MX phosphate is a phosphatase substrate used in the histochemical azo-dye method of phosphatase detection.1 Naphthol AS-MX and Fast Blue BB were used for alkaline phosphatase (AP) activity in conjunction with horseradish peroxidase (POD) and 3-amino-9-ethylcarbazole in chromogenic immunohistochemical double staining techniques.2