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

The selective cleavage of the beta -sitosterol side-chain by free Mycobacte rium sp. NRRL B-3805 cells was used as a model system for the study of solv ent effects in a whole-cell bioconversion in two phase aqueous-organic medi a. This multi-step degradation pathway leads to the production of 4-androst ene-4,17-dione (AD) and 1,4-androstadiene-3,17-dione(ADD) as a minor produc t. In an attempt to correlate the substrate and cell partition effects and solvent hydrophobicity (log P) with biocatalytic activity, 15 carboxylic ac id esters with log P values between 3 and 10 were screened. The results ind icated 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 accumu late in the cells, as these showed a strong affinity towards the organic ph ase. Different solvent/aqueous ratios and hydrodynamic conditions were furt her tested in the solvent systems (phthalates) showing significant biodegra dation activity. The bioconversion rate was generally not much affected by the stirring speed in the employed range (150-300 rpm) but was strongly inf luenced by the aqueous/organic phase ratio. Results suggest that the biocon version takes place at the interphase, its rate being possibly limited by m ass transport inside the organic phase. (C) 2001 Elsevier Science B.V. All rights reserved.