An oxygen supply strategy for the large-scale production of tissue plasminogen activator by microcarrier cell culture

An oxygen supply strategy involving agitation speed and aeration method for the large-scale production of tissue plasminogen activator (TPA) by a microcarrier cell culture was investigated by small-scale model experiments. A preliminary calculation indicated that diffusion limitation of dissolved oxygen (DO) could be caused in a microcarrier sedimentation layer more than 0.5 mm in thickness. Within an agitation speed range above 70 rpm, which was the critical speed for all of the microcarrier beads to remain suspended and thus for avoiding a deficiency of DO, the TPA productivity was higher at a lower agitation speed, while the cell concentration was not affected by the agitation speed. The addition of soluble starch to the culture medium prevented sedimentation of the microcarrier beads, even at the low agitation speed of 20 rpm, resulting in a TPA productivity higher than that at 70 rpm, which was the optimum speed without soluble starch. Use of an air spray system with an optimized air flow rate resulted in a k_La 2.35 times higher than that with simple surface aeration. Increasing the internal pressure of the culture from 0.2 kg/cm² (1209 hPa) to 1.5 kg/cm² (2483 hPa) had no effect on the cell growth but slightly increased the TPA production rates. However, based on the glucose consumption, both the cell and TPA yields were much improved by pressurization. As an optimum mixing and oxygen supply strategy for the production of TPA on a large scale, it is recommended that soluble starch be added to the culture medium to allow the microcarrier suspension to be maintained at a low agitation speed, while keeping a high oxygen transfer rate by means of an air spray system and pressurization.