Involvement of the cell wall ofNocardia restricta on the bioconversions of steroid sapogenins.

The bioconversion of tigogenin, (25R)-5α-spirostan-3β-ol byNocardia restricta to (25R)-4-spirosten-3-one and (25R)-1,4-spirostadien-3-one was strongly increased using protoplasts instead of intact cells. The same effect could be achieved by addingβ-cyclodextrin to the medium. Since 5β-steroid sapogenins pass through the cell wall without hindrance, it can be concluded that the cell wall ofNocardia restricta is a rate-limiting factor only for 5α-isomers.