Expression and purification of a secretory human liver carboxylesterase

Serine-dependent carboxylesterases (E.C.3.1.1.1) are found in a variety of tissues with high activity detected in the liver. Carboxylesterases (CaE) hydrolyze aliphatic and aromatic esters, and aromatic amides; and play an important role in the detoxification of xenobiotic chemicals that contain organophosphate (OP) compounds. The detoxifying ability of CaE is limited by its low concentration in serum where it encounters OP compounds. Studies in our laboratory have shown that a pRc/CMV-hCaE plasmid construct, stably integrated into 293T cells, expresses a human liver CaE in culture. However, the enzyme levels reached a steady state of expression. The goals of this study were to overexpress a functional human liver CaE from a recombinant cDNA in a human cell line, and isolate and purify the recombinant protein. To accomplish these goals, the codon for the terminal leucine (CTG) of the human liver CaE retention signal, HIEL (his-ile-glu-leu), was mutated to that of arginine (CGG). The terminal G of the arginine codon was also mutated to C, to produce a unique ECO47III restriction site to aid in clone selection. The recombinant plasmid, pRc/CMV-mhCaE, was isolated and stably integrated into human 293T cells. Expression of the altered cDNA resulted in secretion of an active CaE to levels in the range of two to five enzyme units per ten milliliters of growth medium. Secretory CaE displayed isoelectric focusing patterns similar to those of the native enzyme with no observable changes in activity. The secreted enzyme was partially purified by hydrophobic interaction chromatography and cibacron blue affinity chromatography. Partial enzyme purification was achieved, and CaE retained a high level of enzymatic activity.