72(P-55) ステロール生合成におけるΔ^<24>オレフィン還元反応の立体化学(ポスター発表の部)

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The final stage of sterol biosynthesis involves reduction of Δ^<24>-olefins, e.g. desmosterol, 24-methyldesmosterol, and 24-ethyldesmosterol, which were established as the biosynthetic precursors of cholesterol, campesterol/dihydrobrassicasterol and sitosterol, respectively. Our continuing interest is the stereochemical course of these biohydrogenation reaction. By similar experimental procedures as the previously described for campesterol/dihydrobrassicasterol biosynthesis, we presently fed chemically prepared [E-Me-^<13>C]-24-ethyldesmosterol and [Z-Me-^<13>C]-24-ethyldesmosterol to cultured cells of Oryza sativa, and analyzed the biosynthesized sitosterol by ^<13>C NMR. It was demonstrated that the E-methyl group of the olefinic precursor becomes the pro-S-methyl on C-25 of sitosterol, implicating an anti addition of hydrogen atoms on the 24-re face and 25-si face of 24(25)-double bond, as observed with campesterol biosynthesis. As for reduction of Δ^<24>-olefin yielding cholesterol, a syn addition in rat liver and an anti addition in Digitalis were proposed 30 years ago. Now isopropylidene (E)-or (Z)-methyl-^<13>C-labelled desmosterol were incubated with a cell free system from the guts of the silkworm larvae and from the rat liver, as well as cultured cells of Oryza sativa. ^<13>C NMR analysis of all the three resulting cholesterols indicates that the C-25 hydrogen was introduced stereospecifically from the si face of the double bond. For elucidation of the stereochemical course of hydrogen addition on the C-24 position, [24-^<13>C, 24-^2H]desmosterol was chemically synthesized and this substrate was fed to the above described three enzyme systems. The biosynthesized cholesterol was analyzed by deuterium-decoupled ^1H, ^<13>C shift correlation NMR, with aid of stereoselectively deuterated standard samples of (24R)-and (24S)-[24-^2H]cholesterol. It was shown that deuterium at C-24 occupies the pro-S position, revealing the stereospecific incorporation of hydrogen atom from the re face of C-24 position of desmosterol. Taken together, it is concluded that addition of hydrogen on 24(25)-double bond of desmosterol takes place in an anti fashion from the re-face of C-24 and the si face of C-25.
天然有機化合物討論会の論文
2002-09-01

72(P-55) ステロール生合成におけるΔ^<24>オレフィン還元反応の立体化学(ポスター発表の部)

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