Process development of transforming phytosterols to steroidal pharmaceutical intermediates by Mycolicibacterium neoaurum with L-carnitine

The process of phytosterol microbial transformation in aqueous phase is the most important pathway to prepare steroidal pharmaceutical intermediates. However, the low solubility of phytosterols and difficult contact between substrate and cells become the main obstacles in practical application. In this study, a comparison of commonly used vegetable oils, surfactants, cosolvents and ionic liquids identified L-carnitine as a novel zwitterionic additive to enhance phytosterol bioconversion by growing and resting cells of Mycolicibacterium neoaurum LLZ2. The addition of L-carnitine had significance in obtaining more biomass and increasing glucose metabolic velocity. Moreover, the surface of cells cultivated with L-carnitine was smoother, which probably enhanced the contact of phytosterols and cells. In other aspects, the dispersion of substrate and several key enzymes involving in the synthesis of steroidal intermediates were not influenced by L-carnitine. Through the optimization of reaction parameters, the total yield of androst-4-ene-3,17-dione (AD) and androst-1,4-diene-3,17-dione (ADD) was improved by 39%. This study provides a green approach for reaction engineering of Mycolicibacterium-mediated phytosterol bioconversion, and new insights into possible mechanism.