Arabidopsis CYP90B1 catalyses the early C-22 hydroxylation of C₂₇, C₂₈ and C₂₉ sterols
2006
Fujita, Satomi | Ōnishi, Toshiyuki | Watanabe, Bunta | Yokota, Takao | Takatsuto, Suguru | Fujioka, Shozo | Yoshida, Shigeo | Sakata, Kanzo | Mizutani, Masaharu
Arabidopsis dwf4 is a brassinosteroid (BR)-deficient mutant, and the DWF4 gene encodes a cytochrome P450, CYP90B1. We report the catalytic activity and substrate specificity of CYP90B1. Recombinant CYP90B1 was produced in Escherichia coli, and CYP90B1 activity was measured in an in vitro assay reconstituted with NADPH-cytochrome P450 reductase. CYP90B1 converted campestanol (CN) to 6-deoxocathasterone, confirming that CYP90B1 is a steroid C-22 hydroxylase. The substrate specificity of CYP90B1 indicated that sterols with a double bond at positions C-5 and C-6 are preferred substrates compared with stanols, which have no double bond at the position. In particular, the catalytic efficiency (k[subscript cat]/K[subscript m]) of CYP90B1 for campesterol (CR) was 325 times greater than that for CN. As CR is more abundant than CN in planta, the results suggest that C-22 hydroxylation of CR before C-5[alpha] reduction is the main route of BR biosynthetic pathway, which contrasts with the generally accepted route via CN. In addition, CYP90B1 showed C-22 hydroxylation activity toward various C₂₇₋₂₉ sterols. Cholesterol (C₂₇ sterol) is the best substrate, followed by CR (C₂₈ sterol), whereas sitosterol (C₂₉ sterol) is a poor substrate, suggesting that the substrate preference of CYP90B1 may explain the discrepancy between the in planta abundance of C₂₇/C₂₈/C₂₉ sterols and C₂₇/C₂₈/C₂₉ BRs.
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