The characteristics of an extracellular poly(3-hydroxyoctanoate) (PHO) depolymerase purified from the marine isolate Pseudomonas luteola M 13-4 were elucidated. The enzyme consisted of a monomeric subunit, having a molecular mass of 28 kDa and isoelectric point of 6.0. The optimum reaction pH and temperature were 10.0 and 40 degrees C, respectively. Its hydrolyzing activity was significantly inhibited by phenylmethylsulfonyl fluoride (PMSF), suggesting the involvement of a serine as an active site amino acid. The enzyme was able to hydrolyze various types of medium-chain-length poly(3-hydroxyalkanoates) (MCL-PHAs) as well as various chain-length p-nitrophenyl (PNP) esters of fatty acids. The amino acid sequence of the tryptic peptide contained a lipase box pentapeptide sequence, G-I-S-S-G. The N-terminal amino acid sequence of the enzyme exhibited 60-68% similarity to those of other MCL-PHA depolymerases. (c) 2005 Elsevier Inc. All rights reserved.