7 オカダ酸の生合成研究 : CID-MS/MSを用いた酸素起源の解明(口頭発表の部)

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Okadaic acid (1) is a polyether compound produced by the marine dinoflagellate Prorocentrum lima. Although its biosynthesis attracts considerable attention since the carbon skeleton has been shown to be synthesized via an unusual route, a limited amount of information is available for the formation of its ether rings. In this study, P. lima was cultured in the presence of N_2 /^<18>O_2 (4: 1), 1mM [^<18>O_2] AcONa and 17% H_2^<18>O, respectively, and a few microgram of 1 isolated from the extract of each cultured cells was subjected to collision-induced dissociation tandem mass spectrometry (CID-MS/MS) to elucidate the ^<18>O-incorporation pattern. The extensive determination of ^<18>O/^<16>O ratios for each product ion bearing the different numbers of incorporated ^<18>O atoms resulted in the complete assignment of the labeled positions with the accurate isotope ratios. These incorporation patterns from ^<18>O_2 and [^<18>O_2] acetate suggest that cyclization of a β-epoxide intermediate at C22-C23 yields ether rings C, D, and E, and the terminal carboxylic acid is formed by Baeyer-Villiger oxidation. A labeling pattern shown by the H_2^<18>O experiments implies that oxygen atoms of water are incorporated via hydration of carbonyl groups, since labeled oxygen functionalities are mostly derived from carbonyl precursors. However, the great difference in the incorporation ratios were observed among them; a hydroxy group at C24 was labeled at 19% while the incorporation in the ether oxygen of ring E was less than 1%. These facts suggest that oxygen incorporation from H_2^<18>O may provide information as to when the reduction of carbonyl groups to hydroxyls takes place in the course of the polyketide biosynthesis; i.e., which enzymes, polyketide synthases (PKSs) or post-PKS enzymes, are implicated in the reaction.
天然有機化合物討論会の論文
1998-08-31

7 オカダ酸の生合成研究 : CID-MS/MSを用いた酸素起源の解明(口頭発表の部)

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