Whole-cell bioconversion of β-sitosterol in aqueous–organic two-phase systems

The selective cleavage of the β-sitosterol side-chain by free Mycobacterium sp. NRRL B-3805 cells was used as a model system for the study of solvent effects in a whole-cell bioconversion in two phase aqueous–organic media. This multi-step degradation pathway leads to the production of 4-androstene-4,17-dione (AD) and 1,4-androstadiene-3,17-dione (ADD) as a minor product. In an attempt to correlate the substrate and cell partition effects and solvent hydrophobicity (log P) with biocatalytic activity, 15 carboxylic acid esters with log P values between 3 and 10 were screened. The results indicated that the toxicity of the tested solvents in this system could not be correlated to their log P, but seemed to depend on their ability to accumulate in the cells, as these showed a strong affinity towards the organic phase. Different solvent/aqueous ratios and hydrodynamic conditions were further tested in the solvent systems (phthalates) showing significant biodegradation activity. The bioconversion rate was generally not much affected by the stirring speed in the employed range (150–300 rpm) but was strongly influenced by the aqueous/organic phase ratio. Results suggest that the bioconversion takes place at the interphase, its rate being possibly limited by mass transport inside the organic phase.