Effect of Cholinium Amino Acids Ionic Liquids As Cosolvents on the Bioconversion of Phytosterols by Mycobacterium sp. Resting Cells

Androst-4-ene-3,17-dione (AD) is an important intermediate of steroid drugs, and the microbial side-chain cleavage of sterols to produce AD by Mycobacterium sp. has attracted much attention. Unfortunately, a low yield of this reaction due to the extremely poor solubility of sterols in aqueous medium is a critical problem. Ionic liquid (IL) as a promising alternative to organic solvents is deemed to increase the solubility of hydrophobic substrates; however, the tolerance of microorganisms to ILs is limited and biocompatible ILs are desperately needed. In this work, cholinium amino acids ([Ch][AA]) ILs were first introduced as cosolvents for the whole-cell bioconversion of phytosterols to produce AD. The physicochemical properties of these novel ILs were investigated on the basis of their structures, and the solubility of phytosterols and AD in [Ch][AA] ILs aqueous solution was determined. The results indicated that moderate cell membrane permeability was obtained by [Ch][AA] ILs, and [Ch][Asp], [Ch][Glu], and [Ch][Gln] showed proper biocompatibility to Mycobacterium sp. MB 3683 cells among 18 kinds of [Ch][AA] ILs. The highest production of AD was achieved in 1% (v/v) [Ch][Asp], which was about twice as high as that in the buffer. Therefore, [Ch][AA] ILs are potential cosolvents in steroid biotransformation to promote medium engineering for a sustainable environment.