Mechanism of the transmethylation reaction by S-adenosylmethionine: stereochemistry of hydride migration from C-24 to C-25 in the biosynthesis of poriferasterol in the crysophyte Ochromonas malharnensis

Abstract

Administration of sodium [2-²H₃]acetate to the crysophyte Ochromonas malhamensis yielded [²H₂₂]poriferasterol, from which, by chemical degradation, the C(24)-C(29) fragment of the side chain was obtained as (2S,3R)-2-[²H₂]methyl[1,1,1,2-²H₄]pentan-3-ol. ²H N.m.r. analysis of this deuteriated compound shows that the 1,2-hydride migration from C-24 to C-25 engages the si-face of the Δ²⁴-sterol double-bond, and suggests that S-adenosylmethionine approaches the same double bond from the re- face.