Indirubin

Indigo and indirubin are derived from indoxyl molecules, which generally occur as indoxyl glycosides in woad (Isatis tinctoria L.) and other indigo-producing plants. Indoxyl glycosides are biosynthesized from indole via 3-hydroxylation to form indoxyl, followed by one or more glycosylations. Enzymes that attach and remove sugars to and from indoxyl have already been isolated and characterized, while enzymes that convert indole into indoxyl in plants have remained elusive, until the identification of P450s and flavin-containing monooxygenases that hydroxylate indole. A P450 gene from woad (named CYP71B102) was heterologously expressed in E. coli, resulting in the formation of indigo and indirubin, as well as isatin and 2-oxindole, which along with indoxyl are putative precursors of indirubin. The addition of either isatin or 2-oxindole to the recombinant E. coli reduced the levels of indigo and increased the amount of indirubin, whereas coexpression of CYP71B102 with isatin hydroxylase (which degrades isatin) increased the levels of indigo and decreased the amount of indirubin, albeit slightly. The results suggest that CYP71B102 hydroxylates indole at both the 2- and 3- positions to produce 2-oxindole and indoxyl, respectively, and that the coupling of indoxyl with either 2-oxindole or isatin forms indirubin, while dimerization of indoxyl forms indigo. This P450 gene is thus likely involved in the biosynthesis of indirubin in woad, as well as the formation of indigo and its glycosidic precursors, even if other types of enzymes, such as flavin-containing monooxygenases, may be involved in indole hydroxylation in other indigo-producing plants.