Dehydrogenation of ribitol with Gluconobacter oxydans: production and stability of L-ribulose

l-Ribulose is an important chiral lead molecule used for the synthesis of, among others, l-ribose, a high-value rare sugar used in the preparation of antiviral drugs. These drugs--nucleoside-analogues--gain importance in the treatment of severe viral diseases, like those caused by the HIV or hepatitis virus. In this study, factors that may have an impact on l-ribulose production with Gluconobacter oxydans and on the stability of l-ribulose were investigated. A bioconversion-type process, using washed resting cells, was chosen to produce l-ribulose from ribitol. In this process, the cell production and bioconversion phase were separated. The former was first optimized and a maximum cell mass of 1.5 g CDWL(-1) could be produced. For the bioconversion phase, the aeration level of the system proved to be one of the most critical factors; a maximal production rate of 15.7 g L(-1)h(-1) or 5.9 g(g CDW)(-1)h(-1) of l-ribulose could be reached. Furthermore, resting cells were found capable of completely converting ribitol solutions of up to 300 g L(-1) within 30 h, although the kinetics indicated a rather low affinity of the dehydrogenase enzymes for the substrate.