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J. Biol. Chem., Vol. 276, Issue 35, 33165-33174, August 31, 2001

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Mutation of Residues 423 (Met/Ile), 444 (Thr/Met), and 506 (Asn/Ser) Confer Cholesteryl Esterase Activity on Rat Lung Carboxylesterase
SER-506 IS REQUIRED FOR ACTIVATION BY cAMP-DEPENDENT PROTEIN KINASE^*

Timothy J. Wallace, Ehab M. Kodsi, Timothy B. Langston, Mervat R. Gergis, and William M. Grogan

From the Department of Biochemistry and Molecular Biophysics, School of Medicine, Virginia Commonwealth University, Richmond, Virginia 23298-0614

Site-directed mutagenesis is used to identify amino acid residues that dictate reported differences in substrate specificity between rat hepatic neutral cytosolic cholesteryl ester hydrolase (hncCEH) and rat lung carboxylesterase (LCE), proteins differing by only 4 residues in their primary sequences. Beginning with LCE, the substitution Met⁴²³  Ile⁴²³ alone or in combination with other mutations increased activity with p-nitrophenylcaprylate (PNPC) relative to more hydrophilic p-nitrophenylacetate (PNPA), typical of hncCEH. The substitution Thr⁴⁴⁴  Met⁴⁴⁴ was necessary but not sufficient for expression of cholesteryl esterase activity in COS-7 cells. The substitution Asn⁵⁰⁶  Ser⁵⁰⁶, creating a potential phosphorylation site, uniformly increased activity with both PNPA and PNPC, was necessary but not sufficient for expression of cholesteryl esterase activity and conferred susceptibility to activation by cAMP-dependent protein kinase, a property of hncCEH. The 3 mutations in combination were necessary and sufficient for expression of cholesteryl esterase activity by the mutated LCE. The substitution Gln¹⁸⁶  Arg¹⁸⁶ selectively reduced esterase activity with PNPA and PNPC but was not required for cholesteryl esterase activity. Homology modeling from x-ray structures of acetylcholinesterases is used to propose three-dimensional models for hncCEH and LCE that provide insight into the effects of these mutations on substrate specificity.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EMBL Data Bank with accession number(s) L46791 and L81144.

The amino acid sequences reported in this paper have been submitted to the Swiss Protein Database under Swiss-Prot accession numbers 1MAA, 1AKN, and 1EA5A.

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