Purification of human monocyte-specific esterase (MSE): molecular and kinetic characteristics

Biology of monocyte-specific esterase

The monocyte-specific (carboxyl)esterase (MSE) is a marker enzyme which is well-known to hematologists as its detection is part of the traditional cytochemical stainings of leukemia cells. There are a variety of synonyms for MSE among hematologists and biochemists. Biochemically, MSE is well-characterized, but should be discerned from other esterases with similar or identical substrate specificities and other features. Intensive analysis of normal and malignant hematopoietic cells and leukemia-lymphoma cell lines using isoelectric focusing established the specificity of this enzyme for monocytes and related cells, hence its designation as monocyte-specific esterase. Cloning of the gene led to its molecular characterization and provided new opportunities to examine MSE expression also at the RNA level which confirmed the monocyte/macrophage specificity. The availability of the gene sequences of various serine esterases and lipases which also hydrolyze ester bonds allowed for the identification of identical isolates from different tissues and the construction of an unrooted dendrogram based on sequence homologies of 22 enzymes. The detailed regulation of the gene and the functional role of MSE have remained largely unknown as of yet. However, DNA binding sites for various transcription factors have already been detected. Some evidence suggests involvement in physiological detoxification processes and in the immune defense against tumor cells. A more thorough understanding of the in vivo function of this truly unique enzyme should be helped by characterizing the signals and signal transduction mechanisms which lead to MSE expression.

Induction of carboxylesterase isozymes in Bombyx mori by E. coli infection

Many proteins, including antibacterial peptides in the hemolymph, are induced by bacterial infections. We found two bacterially inducible carboxylesterases (CEs) in the hemolymph of the silkworm, Bombyx mori. CEs Est-1 and 2 were induced by lipopolysaccharide injection after 6 hours as well as E. coli infection. We found that bacterially inducible CEs clearly differed from noninducible CEs, including juvenile hormone esterases, in pI values, migration on analytical native PAGE, and inhibitor sensitivity. We are now studying the features and functions of these CEs.

Esterase isoenzyme profiles of 255 leukemia-lymphoma cell lines from all hematopoietic cell lineages

Carboxylic esterase isoenzymes isolated from a panel of well-characterized continuous human leukemia-lymphoma cell lines were separated by isoelectric focusing. Typical isoenzyme patterns designated Mono 1/Mono 2 (for monocyte-associated), My 1/My 2 (for myeloid or myeloma), Lym 1/Lym 2 (for lymphoid) and Und (for undifferentiated) could be reproducibly discerned. The Mono patterns contained one unique isoenzyme encoded by the monocyte-specific esterase gene. This comparative analysis of 255 leukemia-lymphoma cell lines covered the major cell lineage that are affected by hematological neoplasias. The results showed that (except for myelomas) lymphoid-derived malignancies, both leukemias and lymphomas, expressed primarily the Und and Lym esterase isoenzyme profiles. In contrast, myeloid leukemia cells and the related erythroid and megakaryocytic cell lines displayed mainly the My patterns. The Mono patterns were detected predominantly in monocyte-derived leukemias. As the B-lymphocytic hierarchy progresses from pre B-cells via B-cells to plasma cells, number and intensity of the isoenzymes increased as well from the Und pattern to the My isoenzyme profile. Hodgkin's disease and anaplastic large cell lymphoma lines displayed heterogenous isoenzyme profiles consistent with their heterogenous cellular origin. The present study using continuous leukemia-lymphoma cell lines as model systems provides a biochemical characterization of different hematopoietic cell lineages and stages of differentiation.

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.