A simple structured model for maintenance, biomass formation, and ajmalicine production by nondividing Catharanthus roseus cells

The stoichiometry of maintenance and carbo- hydrate storage as well as ajmalicine production kinetics of non-dividing Catharanthus roseus cells in the second stage of a two-stage batch process were investigated. For the mathematical description of these processes, a simple structured model with 5 parameters is proposed. In the model the biomass is divided in two compart- ments: active biomass and storage carbohydrates. In in- duction medium (standard medium without phosphate, nitrogen and hormones), biomass formation, glucose consumption, and CO2 production appeared to be con- stant in time. Therefore, it is assumed that the active biomass level is constant. The maintenance coefficient mS, and the yield of storage carbohydrates on glucose YSC were optimized by fitting the model on experimental data: 0.003 C-mol/C-mol/h and 0.82 C-mol/C-mol, respec- tively. Production kinetics were incorporated in this model and related to the active biomass fraction. The maximum specific ajmalicine production rate qp max was fitted on the data: 7.5 µmol/C-mol/h. The model was tested at several different experimental conditions, and proved to describe the experimental results adequately. An independent experiment at a very high cell density in order to obtain maximum product formation was used to validate the model. It provided a satisfactory description of the results, but the final ajmalicine concentration (198 µmol/L after 18 days) was lower than the calculated maximum, due to accumulation of inhibiting gaseous metabolites. © 1999 John Wiley & Sons, Inc. Biotechnol Bio- eng 66: 147-157, 1999.