吉本 尚子

Marasmin [S-(methylthiomethyl)-L-cysteine-4-oxide] is a pharmaceutically valuable sulfur-containing compound produced by the traditional medicinal plant, Tulbaghia violacea. Here, we report the identification of an S- oxygenase, TvMAS1, that produces marasmin from its corresponding sulfide, S-(methylthiomethyl)-L-cysteine. The amino acid sequence of TvMAS1 showed high sequence similarity to known flavin-containing S-oxygenating monooxygenases in plants. Recombinant TvMAS1 catalyzed regiospecific S-oxygenation at S4 of S-(methylthiomethyl)-L-cysteine to yield marasmin, with an apparent Km value of 0.55 mM. TvMAS1 mRNA accumulated with S-(methylthiomethyl)-L-cysteine and marasmin in various organs of T. violacea. Our findings suggest that TvMAS1 catalyzes the S-oxygenation reaction during the last step of marasmin biosynthesis in T. violacea.

S-Alk(en)ylcysteine sulfoxides (CSOs), such as methiin, alliin, and isoalliin, are health-beneficial natural products biosynthesized in the genus Allium. Here, we report the induction of multiple callus tissue lines from three Allium vegetables, onion (A. cepa), Welsh onion (A. fistulosum), and Chinese chive (A. tuberosum), and their ability to accumulate CSOs. Callus tissues were initiated and maintained in the presence of picloram and 2-isopentenyladenine as auxin and cytokinin, respectively. For all plant species tested, the callus tissues almost exclusively accumulated methiin as CSO, while the intact plants contained a substantial amount of isoalliin together with methiin. These results suggest that the cellular developmental conditions and the regulatory mechanisms required for the biosynthesis of methiin are different from those of alliin and isoalliin. The methiin content in the callus tissues of onion and Welsh onion was much higher compared to that in the intact plants, and its cellular concentration could be estimated as 1.9–21.7 mM. The activity of alliinase that degrades CSOs in the callus tissues was much lower than that of the intact plants for onion and Welsh onion, but at similar levels as in the intact plants for Chinese chive. Our findings that the callus tissues of onion and Welsh onion showed high methiin content and low alliinase activity highlighted their potential as a plant-based system for methiin production.

一般的に含硫化合物には化学反応性が高いものが多く，天然の含硫化合物の中にも特異な生物活性を示すものが多く存在する．アブラナ科植物が含有するグルコシノレートや，ヒガンバナ科ネギ属植物が含有するアリイン類は，これらの植物の特徴的な香味の原因となる含硫化合物である．近年の研究により，グルコシノレートが癌の予防や改善に，アリイン類が癌や循環器系疾患に加えて加齢による認知機能低下の予防や改善に役立つことが示された．本稿では，グルコシノレートとアリイン類の健康機能発揮のメカニズムと，これら含硫化合物を含む植物の生薬やサプリメントとしての利用状況について概説する．

ケシが生合成するモルヒナン型アルカロイドのうち，モルヒネはがんや外傷による疼痛の緩和に用いられる天然オピオイドとして，テバインは半合成オピオイドであるオキシコドンやオピオイド受容体拮抗薬であるナロキソン等の製造原料として，極めて重要な化合物である．気象災害や病害によるケシ収穫量減少に伴う医薬品の供給不足を避けるため，遺伝子組換え微生物を用いて，モルヒネやテバインを安定で効率的に生産する技術の開発が期待されている．今回，モルヒネ生合成において非酵素的に進むと考えられていたテバイン合成反応を触媒する酵素が同定され，効率的なテバインの異種生物生産が可能になったので紹介する．