SUBCELLULAR FRACTIONATION STUDIES INDICATE AN INTRACELLULAR-LOCALIZATION FOR HUMAN MONOCYTE SPECIFIC ESTERASE (MSE)

Human monocyte-specific esterase (MSE) is one of the few haemopoietic cell enzymes that show absolute lineage restriction. Although the function of MSE has yet to be deduced, its potential role in tumour cell killing has stimulated particular interest. Knowledge of subcellular localization of MSE is fundamental to understanding its function and, in this context, it is widely believed that MSE is expressed as a plasma membrane ectoenzyme; a contention that is largely based upon experiments which examined fixed cells by ultrastructural cytochemistry. However, as recent molecular studies of human MSE indicate, a number of inconsistencies between its structure and a membrane localization, we applied the techniques of phase separation in the non-ionic detergent Triton X-114 and differential centrifugation to further investigate whether this particular esterase species is membrane-bound or associated with an intracellular organelle. These studies provide strong evidence that MSE is in fact a soluble intracellular enzyme that is almost certainly located within the lumen of the endoplasmic reticulum.