Identification and localization of two membrane-bound esterases from Escherichia coli

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Abstract
Hydrolytic activities of isolated membrane fractions of E. coli against chromogenic substrates, p-nitrophenyl ester and .beta.-naphthyl ester derivatives of N-substituted amino acids, were investigated by spectrophotometric and electrophoretic methods. Although detergents were absolutely necessary for the solubilization of enzymes, the amount of solubilized activities was increased by adding salt, such as NaCl or KCl. Two esterases were identified and separated by PAGE [polyacrylamide gel electrophoresis] and by chromatography of the solubilized proteins in the presence of detergent. One hydrolyzed the alanine derivatives preferentially, whereas the other was mainly active on phenylalanine derivatives. Only the 1st was inactivated by DFP, a serine hydrolase inhibitor. Whereas the chymotrypsin-like enzyme was equally distributed between the inner and the outer membrane, the alanine activity was only detected in the inner membrane. They were both resistant to extraction with high salt concentrations, indicating their integral association with membranes. A study of the accessibility of these enzymes to their substrate in membrane vesicles with known polarity suggests that both alanine and phenylalanine activities are localized near the external surface of the cytoplasmic (inner) membrane. The phenylalanine activity (chymotrypsin-like enzyme) appears to be deeply buried inside the outer membrane. Because of its insensitivity to DFP, this esterase seems to be distinct from the previously isolated periplasmic endopeptidase, protease I, which is also a chymotrypsin-like enzyme.