Maintenance of cell viability in the biotransformation of (−)-carveol with whole cells of Rhodococcus erythropolis

Abstract

Whole cells of Rhodococcus erythropolis DCL14 present carveol dehydrogenase (CDH) activity, which allows them to stereoselectively carry out the oxidation of the (+)-cis and (−)-trans-carveol to (+)- and (−)-carvone, respectively [1]. When a diastereomeric mixture of (−)-carveol was supplied for biotransformation, the (−)-trans-carveol was converted to (−)-carvone. When the cells grow on limonene or cyclohexanol the major activity is NAD-dependent. The relatively low water solubility of carveol and carvone was overcome through the implementation of an organic:aqueous system. The prolonged productivity of such a system depends on cell viability, since viable cells are naturally able to regenerate the co-factor. Fluorescence microscopy was used to off-line monitor cell viability during the time course of the biotransformation.n-Dodecane was the solvent that allowed the highest retention of both cell activity and viability. The most adequate phase ratio was 1:5, at which, for ODs higher than 0.57, a stable emulsion is formed. At an OD of 0.46 only half of the solvent was emulsified. Loss of viability increased with the OD, probably due to oxygen depletion. The maximum specific production rate was obtained at an initial carveol concentration of 125mM. At this scale (60ml flasks) the best aeration rate was 0.01vvm, both with regard to viability and stability of the emulsion. Carvone was found to be toxic, causing cell death at concentrations above 50mM.