We previously purified a new esterase from the thermoacidophilic eubacterium Bacillus acidocaldarius whose N-terminal sequence corresponds to an open reading frame (ORF3) reported to show homology with the mammalian hormone-sensitive lipase (HSL)-like group of the esterase\lipase family. To compare the biochemical properties of this thermophilic enzyme with those of the homologous mesophilic and psychrophilic members of the HSL group, an overexpression system in Escherichia coli was established. The protein, expressed in soluble and active form at 10 mg\l E. coli culture, was purified to homogeneity and characterized biochemically. The enzyme, a 34 kDa monomeric protein, was demonstrated to be a Bd-type carboxylesterase (EC 3.1.1.1) on the basis of substrate specificity and the action of inhibitors. Among the p-nitrophenyl (PNP) esters tested the best substrate was PNP-exanoate with K m and k cat values of 11p2 µM (meanpS.D., n l 3) and 6610p880 s −" (meanpS.D., n l 3)

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