A Dioxygenase Catalyzes Steroid 16α-Hydroxylation in Steroidal Glycoalkaloid Biosynthesis. Issue 1 (28th July 2017)

• Record Type: Journal Article
• Authors: Nakayasu, Masaru; Umemoto, Naoyuki; Ohyama, Kiyoshi; Fujimoto, Yoshinori; Lee, Hyoung Jae; Watanabe, Bunta; Muranaka, Toshiya; Saito, Kazuki; Sugimoto, Yukihiro; Mizutani, Masaharu
• Is Part Of: Plant physiology. Volume 175:Issue 1(2017)
• Page Start: 120
• Page End: 133
• Publication Date: 2017-07-28
• Subjects: Plant physiology -- Periodicals; Botany -- Periodicals; Periodicals; Electronic journals; 571.2
• DOI: 10.1104/pp.17.00501
• Languages: English
• ISSNs: 0032-0889

Abstract

Abstract : The 2-oxoglutarate-dependent dioxygenase 16DOX catalyzes steroid 16α-hydroxylation in the steroidal glycoalkaloid (SGA) pathway and is a suitable target for controlling toxic SGA levels in potato. Abstract: Steroidal glycoalkaloids (SGA s) are toxic specialized metabolites that are found in the Solanaceae. Potato ( Solanum tuberosum ) contains the SGA s α-solanine and α-chaconine, while tomato ( Solanum lycopersicum ) contains α-tomatine, all of which are biosynthesized from cholesterol. However, although two cytochrome P450 monooxygenases that catalyze the 22- and 26-hydroxylation of cholesterol have been identified, the 16-hydroxylase remains unknown. Feeding with deuterium-labeled cholesterol indicated that the 16α- and 16β-hydrogen atoms of cholesterol were eliminated to form α-solanine and α-chaconine in potato, while only the 16α-hydrogen atom was eliminated in α-tomatine biosynthesis, suggesting that a single oxidation at C-16 takes place during tomato SGA biosynthesis while a two-step oxidation occurs in potato. Here, we show that a 2-oxoglutarate-dependent dioxygenase, designated as 16DOX, is involved in SGA biosynthesis. We found that the transcript of potato 16DOX ( St16DOX ) was expressed at high levels in the tuber sprouts, where large amounts of SGA s are accumulated. Biochemical analysis of the recombinant St16DOX protein revealed that St16DOX catalyzes the 16α-hydroxylation of hydroxycholesterols and that (22 S )-22, 26-dihydroxycholesterol was the best substrate among the nine compounds tested. St16DOX -silenced potato plants contained significantly lower levels of SGA s, and a detailed metabolite analysis revealed that they accumulated the glycosides of (22 S )-22, 26-dihydroxycholesterol. Analysis of the tomato 16DOX ( Sl16DOX ) gene gave essentially the same results. These findings clearly indicate that 16DOX is a steroid 16α-hydroxylase that functions in the SGA biosynthetic pathway. Furthermore, St16DOX silencing did not affect potato tuber yield, indicating that 16DOX may be a suitable target for controlling toxic SGA levels in potato.