Dynamic assay of enzyme activities in single cells by flow cytometry

Three enzymes in single cells were assayed dynamically by flow cytometry using four fluorogenic substrates. Acid phosphatase was determined with 7-bromo-3-hydroxy-2-naphtho-o-anisidine (naphthol AS-BI) phosphate and 4-methylumbelliferone (MU) phosphate, neutral esterase with fluorescein diacetate, and lactic dehydrogenase with NAD-sodium lactate. Fluorescence measurements obtained with the flow cytometer were converted into relative specific enzyme activities for single cells with molar fluorescence coefficients determined with a spectrofluorometer. Specific activities obtained from spectrofluorometric data were compared with activities calculated from flow cytometeric data. Flow cytometric assays gave lower specific single cell activities for 4-methylumbelliferone phosphate hydrolysis and for lactic dehydrogenase than did similar assays by standard spectrofluorometry. Product diffusion may be the greatest cause for this discrepancy.