A soil-isolated bacterium (strain B4) was identified as a species of Bordetella and deposited with the China General Microbiological Culture Collection (code, CGMCC 2229). The bacterium grew in a mineral medium, on cholesterol as a sole source of carbon and energy. Only one metabolite of cholesterol was accumulated in detectable amounts during the strain growth. It was identified as 4-cholesten-3-one. Cholesterol oxidase (COD) (EC 1.1.3.6), which catalyzes cholesterol into this metabolite, was evidenced from the strain. The conditions of the bacterium growth were optimized for extracellular enzyme production, which then reached around 1700 UL-1 within 24 h culturing. The enzyme was purified from the spent medium of the strain to homogeneity on SDS-PAGE, and characterized. Its molecular mass, as estimated by this technique, was 55 kDa. COD showed an optimum activity at pH 7.0. It was completely stable at pH 5.0 and 4 degrees C for 48 h, and retained 80% at least of its initial activity at pH 4.0 or at a pH of 6.0-10.0. The optimum temperature for its reaction was 37 degrees C. The thermal stability of COD was appreciable, as 90% or 80% of its initial activity was recovered after 1 h or 2 h incubation at 50 degrees C. Ag+ or Hg+ at 1 mM, was inhibitor of COD activity, while Cu2+ at the same concentration, was activator. The COD K-m, determined at 37 degrees C and pH 7.0, was 0.556 mM. The enzyme was stable at pH 7.0 and 37 degrees C during 24h mechanical shaking in the presence of 33% (v/v) of either of the solvents, dimethylsulfoxide, ethyl acetate, butanol, chloroform, benzene, xylene or cyclohexane. (C) 2010 Elsevier Ltd. All rights reserved.