Cytochemical reaction for esterase using different substrates allows the distinction between cells of the monocyte lineage and the cells of the neutrophil lineage. Apart from the naphthol chloroacetate esterase reaction, whose reliability is comparable with that of the peroxidase reaction, 1-naphthyl-acetate esterase reaction is the most suitable for identifying monoblastic types of leukemia.
Leukocyte esterase hydrolyzes a synthetic substrate and an ester that is a derivate of naphthalene. Released naphthol rapidly couples with a diazonium salt present in the staining mixture, resulting in a brightly red-brown or black-brown colored precipitate at or near the site of the enzyme activity.
Chloracetate esterase has an optimum pH between 7.0 and 7.6 and is insensitive to fluorid inhibition. It is present in the non-specific granules of promyelocytes and of neutrophils.
Nonspecific esterases react best at pH between 6.0 and 6.3, and are inhibited by fluorides. Alpha-naphthyl acetate esterase is strongest in monocytes, macrophages, megakaryocytes, and platelets. It is also positive in basophils and plasma cells. Sodium fluoride, inhibits the activity in monocytes, megakaryocytes, platelets, and plasma cells but not that in lymphocytes and granulocytes.