Abstract
Baicalein, wogonin, and their glycosides are major bioactive compounds found in the medicinal plant Scutellaria baicalensis Georgi. These flavones can induce apoptosis in a variety of cancer cell lines but have no effect on normal cells. Furthermore, they have many additional benefits for human health, such as anti-oxidant, antiviral, and liver-protective properties. Here, we report the isolation and characterization of two CYP450 enzymes, SbCYP82D1.1 and SbCYP82D2, which function as the flavone 6-hydroxylase (F6H) and flavone 8-hydroxylase (F8H), respectively, in S. baicalensis. SbCYP82D1.1 has broad substrate specificity for flavones such as chrysin and apigenin and is responsible for biosynthesis of baicalein and scutellarein in roots and aerial parts of S. baicalensis, respectively. When the expression of SbCYP82D1.1 is knocked down, baicalin and baicalein levels are reduced significantly while chrysin glycosides accumulate in hairy roots. SbCYP82D2 is an F8H with high substrate specificity, accepting only chrysin as its substrate to produce norwogonin, although minor 6-hydroxylation activity can also be detected. Phylogenetic analysis suggested that SbCYP82D2 might have evolved from SbCYP82D1.1 via gene duplication followed by neofunctionalization, whereby the ancestral F6H activity is partially retained in the derived SbCYP82D2.
Highlights
Scutellaria baicalensis Georgi or Huangqin is a medicinal plant used widely in China and many other Asian countries
To identify genes that may hydroxylate chrysin in Scutellaria and might be involved in baicalein biosynthesis, we screened for contigs from RNAsequencing (RNA-seq) databases of hairy roots and flowers (Zhao et al, 2016b) encoding proteins that were annotated as CYP71D or CYP82D, and performed BLAST searches using CYP71D9 and CYP82D33 as bait
We have shown previously that a newly evolved pathway is involved in the biosynthesis of chrysin, which is decorated by an flavone 6-hydroxylase (F6H) to produce baicalein or by an flavone 8-hydroxylase (F8H) and an 8-OMT (8-O-methyltransferase) to make wogonin
Summary
Scutellaria baicalensis Georgi or Huangqin is a medicinal plant used widely in China and many other Asian countries. The pharmacological activities of S. baicalensis have been attributed mainly to the large amounts of 40-deoxyflavones, which accumulate in roots (root-specific flavones [RSFs]): baicalin, wogonoside, and their aglycones, baicalein and wogonin (for structures, see Supplemental Table 1) (Makino et al, 2008; LiWeber, 2009). These flavones are reported to have various benefits for human health, such as anti-fibrotic activity in the liver and anti-cancer properties (Gao et al, 2011; Yang et al, 2012).
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