Dehydrogenation and dealkylation of various sterols byTetrahymena pyriformis

Abstract

AbstractSterols are not found inTetrahymena pyriformis when this protozoan is grown in a medium free from exogenous sterols; instead, the principal solid alcohol that can be isolated from the organism is tetrahymanol. a pentacyclic triterpenoid alcohol with an unusual structure. The biosynthesis of tetrahymanol has been shown by appropriate labeling studies to involve a direct, nonoxidative, proton‐initiated cyclization of squalene rather than the more commonly found type of mechanism involving squalene 2,3‐oxide as an intermediate. In contrast, whenT. pyriformis is incubated with any one of a wide variety of added sterols, the biosynthesis of tetrahymanol is inhibited and the added sterol is accumulated by the organism and, in most cases, is converted metabolically into one or more other sterols. Four different types of transformation have been observed: the introduction of Δ5, Δ7 and Δ22 double bonds, and the removal of ethyl groups, but not methyl groups, from C‐24.