Origin of the 28-pro-R and 28-pro-S hydrogens in the biosynthesis of poriferasterol in Ochromonas malhamensis

It is shown, by 2H n.m.r. spectroscopy, that in the biosynthesis of poriferasterol in Ochromonas malhamensis the hydrogen atom at C-28 arising from S-adenosylmethionine assumes the 28-pro-S position, whereas the hydrogen atom coming from the reducing system assumes the 28-pro-R position.