Improved methodology for intracellular enzyme reaction and inhibition kinetics by flow cytometry

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Abstract
Flow cytoenzymology is the determination of enzyme activities or concentrations in single intact cells. Using the flow cytometer built and designed in our laboratory and recent modifications to hardware and software, we have developed an improved dynamic flow cytoenzymological procedure for the assay of cellular enzyme kinetics. The reaction mixture is sampled continuously, and the computer clock incorporates time as a parameter for kinetic determinations. Conditions for cellular esterase analysis were optimized and the rates of hydrolysis of two fluorogenic substrates, fluorescein diacetate (FDA) and 4‐methylumbelliferone acetate (MUA), by esterases in EMT6 mouse mammary tumor cells were studied. Reaction kinetics were characterized, and Km values of 19 and 72 μM were obtained for the hydrolysis of FDA and MUA respectively. The kinetics of the cellular efflux of fluorescein were investigated, and a half‐life of 7.5 min obtained. Enzyme inhibition kinetics were investigated using the competitive substrates p‐nitrophenyl acetate and phenyl acetate, and the carbamoylating agents physostigmine and n‐butyl isocyanate. The latter was particularly potent with an I50 of 4.8 × 10−5 M for FDA hydrolysis compared with 6.5 × 10−3 M for physostigmine. The I50 of 8.8 × 10−5 M for n‐butyl isocyanate inhibition of MUA hydrolysis was similar to that obtained with FDA as substrate. By monitoring FDA and MUA reactions separately and simultaneously, we showed them to act as competitive substrates. A comparison of flow cytoenzymological and conventional spectrofluorimetric analysis was also made, and differences identified in some cases.