Microbial conversions in a liposomal medium. Part 2: Cholesterol oxidation by Rhodococcus erythropolis

The oxidation of cholesterol (5 g l^-1) to cholestenone by Rhodococcus erythropolis ATCC 25544 was investigated in liposomal media made up of multilamellar vesicles (MLVs) and small unilamellar vesicles (SUVs) of phosphatidylcholine. While MLVs practically inhibited the biotransformation, SUVs promoted it at a faster rate and to a higher conversion than those obtained in an aqueous sterol dispersion. The reverse, however, was observed when the same conversion in SUVs was catalysed by free soluble cholesterol oxidase. A lower phospholipid:cholesterol ratio in SUVs enhanced the rate of the microbial conversion, but a negative charge slowed it down. The mechanism of the bioconversion in the liposomal medium was interpreted as a direct interaction between the microbial cells and SUVs. During the first 24 h, the bioconversion proceeded to completion in intact liposomes, and by then Rhodococcus cells had adapted to the phospholipids and could partly metabolize them with extended incubation.