Origin of the Methoxyl Groups in Spirilloxanthin—a Carotenoid synthesized by the Photosynthetic Bacterium Rhodo-spirillum rubrum

Abstract

IT can now be stated with a reasonable assurance that the repeating (isoprene) unit of all terpenoid compounds arises from two-carbon (acetate) units via mevalonic acid (β,δ-dihydroxy-β-methylvaleric acid) or a closely related compound¹. Occasionally, however, additional carbon atoms appear the origin of which cannot readily be referred to an isoprene unit ; this is most marked in the plant sterols and perhaps the best example is ergosterol with its additional methyl substituent at C₂₄. It has been shown that this group arises by means of the transfer of a one-carbon unit from a donor such as active methionine or formyl- or hy droxy methyl-tetra-hydrofolic acid, because formate-¹⁴ C is readily and specifically incorporated into C₂₈ of ergosterol in yeast².