Development of a reaction system for the selective conversion of (-)-trans-carveol to (-)-carvone with whole cells of Rhodococcus erythropolis DCL14

The present article addresses the development of a microbial reaction system for the transformation of carveol to carvone, using whole cells of Rhodococcus erythropolis DCL14. This strain contains a NAD-dependent carveol dehydrogenase (CDH) when grown on limonene or on cyclohexanol. When a mixture of (-)-cis and (-)-trans-carveol is supplied, only (-)-trans-carveol is converted. Thus, besides(-)-carvone, pure (-)-cis-carveol can be obtained as product. Initial experiments were performed batchwise using an aqueous system. (-)-Trans-carveol conversion rate gradually decreased during successive reutilisation batches. After the third reutilisation, activity was completely lost. Cells grown on cyclohexanol showed a slightly higher activity as compared to cells grown on (+)-limonene. A production of 4.3 mu mol (-)-carvone formed per mg protein was achieved. A significant improvement with respect to initial reaction rate and productivity was obtained with aqueous-organic two-phase systems. Using a 5 to 1 buffer/iso-octane system, a 40% increase in the initial rate and a 16-fold increase of the production was observed. A further improvement resulted from increasing the volume of solvent (1 to 1 buffer/dodecane ratio). An initial reaction rate of 26 nmol/(min* mg protein) was observed, while production increased to 208 mu mol (-)-carvone formed per mg protein. As in the single-phase system, reaction rate gradually decreased along the successive cell reutilisation batches. Addition of co-substrates for the regeneration of NAD did not prevent this decay. A simple downstream process was developed for the recovery of carvone and cis-carveol. (C) 2001 Elsevier Science B.V. All rights reserved.