Lineage-Specific Monocytic Esterase, a Distinct Marker for Leukemias of Monocytic Origin: Cytochemical, Isoenzymatic...

Cytochemical staining of "non-specific esterases" is commonly used in the classification of acute leukemias. In the biochemical nomenclature these enzymes represent the "carboxylesterases (EC 3.1.1.1)" and are mostly visualized by a substrate-diazonium dye reaction using alpha-naphthyl acetate which led to the term "alpha-naphthyl acetate esterases (ANAE)". Separation of cell extracts by polyacrylamide gel electrophoresis (PAGE) or isoelectric focusing (IEF) have demonstrated that the carboxylic esterase (system) comprises a number of distinct isoenzymes. Included among these is a distinct isoenzyme (group), defined by IEF with acidic isoelectric points (pI) of approximately 6.0, that is characterized by its strong staining intensity and inhibition by sodium fluoride (NaF). Due to its selective expression in normal and malignant monocytes and its unique plasmalemmal location as an ectoenzyme on monocytes, this isoenzyme (the "monocyte esterase") appears to be specific for cells of monocytic origin. While a number of studies have analyzed the biochemical features of the monocyte esterase, little is known about its physiological function. Examining purified normal cell populations and fresh or cultured leukemia cells, it was found that the expression of the monocyte esterase is, indeed, restricted to cells of monocytoid/macrophage differentiation. Expression of this protein can be induced in-vitro by physiological or pharmacological agents in responsive cell populations which can be triggered to differentiate along the monocyte/macrophage cell axis. Compared with the level of enzymatic activity in normal monocytes, there seems to be an overexpression of monocyte esterase in some leukemic cells. Molecular cloning of the gene encoding this protein, an emphasis on studies of function and further detailed characterization of the biochemistry are future avenues of research.