Elucidating Cysteine-Assisted Synthesis of Indirubin by a Flavin-Containing Monooxygenase

Indirubin is a biologically active compound found in Danggui Longhui Wan, which is a traditional Chinese medicine for chronic myelocytic leukemia. In the biosynthesis of indirubin, the formation of indigo, which is a stereoisomer of indirubin, is a major side reaction. Recent findings have suggested that cysteine supplementation shifts product selectivity from indigo to indirubin. Here, we disclose how cysteine is involved in enhancing the product selectivity in the synthesis of indirubin using a flavin-containing monooxygenase from Methylophaga aminisulfidivorans (MaFMO). First, cysteine reacts with indoxyl to synthesize 2-cysteinylindoleninone, inhibiting the dimerization of indoxyl. Second, the reducing power of cysteine allows MaFMO to additionally hydroxylate indoxyl toward isatin, overcoming the problem in biased distribution of two different precursors. Third, cysteine activates isatin to react with 2-cysteinylindoleninone to form indirubin. Based on this revealed mechanism, indirubin derivatives with different indole ring components were synthesized.