The biological transformation from rifamycin B to rifamycin S was carried out using the living whole cells of $\underline{Humicola spp.}$ (ATCC 20620). This cell was known to possess rifamycin B oxidase catalyzing the oxidative cyclization of rifamycin B to rifamycin 0, which was spontaneously hydrolyzed to rifamycin S in a neutral aqueous milieu.
Since this reaction required the oxygen as a gaseous substrate in addition to the rifamycin B solution in liquid phase, the dual hollow fiber bioreactor was used.
$\underline{Humicola spp.}$ was successfully grown within the region between the silicone tube and three polypropylene hollow fiber membranes.
The production media containing rifamycin B solution were artificially designed for maintaining the viability of cells and thus the time stability of enzyme was much improved. The use of nitrogen deficient medium was desirable for stable reactor operation.
By shifting reaction temperature from 30℃ to 40℃, the initial enzyme activity increased but the stability of enzyme decreased. The reason was that the enzyme activity at 40℃ was higher than that at 30℃, but the viability of cells at 40℃ was lower than that at 30℃ and thus the time stability of enzyme became worse.
In continuous operation, as the space time increased from 0.4hr to 2.3hr, the productivity decreased from 1.15(mmol/L$＼cdot$Hr) to 0.3(mmol/L$＼cdot$hr).
