The enzymatic production of rifamycin S was carried out using immobilized $\underline{Humicola spp.}$ (ATCC 20620) treated with acetone. This cell was known to possess rifamycin B oxidase capable of oxidizing rifamycin B to rifamycin S. This reaction requires oxygen and the substrate in the liquid phase, which constitutes a typical three-phase reaction. Rotating packed disk reactor (RPDR) was used as the reactor. It was found that optimum pH was 7.2 for both free cell and immobilized cell, and optimum temperature was 45-50℃ for free cell and 50-55℃ for immobilized cell. Immobilized cell rarely showed decrease of activity with time, if it was kept at pH 7.2 and below 40℃. It had about 10 hr half-life at 50℃, but at slightly higher temperature, the activity decreased rapidly due to enzyme denaturation. Activity recovery was about 20% for the bead of 3mm diameter in size.
Although this result was not desirable, cellulose acetate bead could be adopted as appropriate packing material for the RPDR due to its high mechanical strength and stability at high temperature.
In reactor operation, it was found that optimum submergence of the disk was about 0.5 and rpm effect was negligible for this reaction. In continuous operation, as the space time increased, the productivity decreased. The steady state was obtained after 4-5 times space time.
