Cholesterol oxidase (COX, E.C.18.104.22.168) catalyses the oxidation of cholesterol to 4-cholestene-3-one with the reduction of oxygen to hydrogen peroxides. COXs are secreted bacterial enzymes that catalyze the first step in the degradation of cholesterol. Some bacteria, such as Mycobacterium, Rhodococcus and Nocardia sp. produce an intracellular form of the enzyme that is membrane bound, while the enzyme from Arthrobacter, Schizopyllum, Streptoverticillium, Brevibacterium and Streptomyces is found in the extracellular fraction. These organisms play important roles in biotransformation and bioconversion of organic compounds. Bioconversion reactions are the subject of increasing interest in the pharmaceutical industry because of the demand for enantiomerically pure compounds. Bioconversion processes that involve enzymatic or microbial biocatalysts, when compared to their chemical counterparts, offer the advantages of high selectivity and mild operating conditions. Bioconversions may involve isolated and purified enzymes directly in free or immobilized form in order to enhance process stability. Medium engineering attempts to enhance the solubility of substrate and remove(s) the inhibition of product simultaneously by adding an inherently biocompatible and non-biodegradable ingredient to bioconversion medium. The extremely poor solubility of cholesterol as a substrate or steroids in aqueous media lowers the transformation rate and increase costs. The methods of enhancing steroid solubility in bioconversion media include substrate derivatization or micronization, ultrasonication or the use of detergents, water miscible co-solvents, cyclodextrins, polymers and liposomal aqueous biphasic media. Steroids like cholesterol are completely soluble in some organic solvents like benzene, toluene and butanol. Biphasic systems where in the microbial cells are present in the aqueous phase and steroids dissolved in the organic phase is considered an ideal system. In the present review article we try to discuss on the solvent tolerant properties and biotransformation capability of cholesterol oxidase producing different organisms of different species and their applications in different fields.
Key words: Keywords: Microbial cholesterol oxidases, biotransformation, bioconversion, organic solvent-tolerance, industrial applications.