Steroid bioconversion in a microemulsion system

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Abstract
The Δ1,2‐dehydrogenation of high concentrations of the steroid ‐methyl‐Reichstein's compound S‐21‐acetate (16MRSA) in a microemulsion system was studied using heat‐dried and thawed Arthrobacter simplex cells as biocatalyst. The microemulsion system consists of an organic phase [75−95% (v/v)] with steroid (1−60 g/Ltot), an aqueous phase [5−25% (v/v)] containing the cells (5−30 g/Ltot), and a neutral surfactant (5−20 g/L organic solvent). Benzene derivatives, which solubilize 16MRSA up to 94 g/L, and phospholipids were used as organic solvents and surfactants, respectively, and menadione was added as an external electron acceptor. Factors affecting the dehydrogenation rate in the microemulsion system were studied. The influences of the 16MRSA and the menadione concentration on the dehydrogenation rate were described by Michaelis‐Menten kinetics, apparent V′max and K′m values of 2.06 g/g dry weight h and 18.9 g/L for 16MRSA and 4.97 g/g dry weight h and 1.91 g/L for menadione being obtained. Optimal menadione concentration was dependent on the steroid concentration was dependent on the steroid concentration used. The reaction was strongly inhibited by high product concentrations. Much higher activities were obtained with the thawed cells than with the dried cells, conversions of 98% being reached within 14−16 h. for 16MRSA and cell dry weight concentrations of 40 and 10 g/L, respectively. Activity retention in a batch stirred tank reactor remained constant during the first 16−24 h of operation and then decreased, depending on the stirring rate; 22 to 65% of the initial reaction rate was obtained after 48 h at stirring rates of 650 and 2000 rpm, respectively.