The steric requirements for sterol inhibition of tetrahymanol biosynthesis

Many naturally occurring sterols are accumulated and metabolized byTetrahymena pyriformis. In most cases, the sterols are desaturated to giveΔ5,7,22-derivatives. Compounds with an ethyl, but not with a methyl, substituent at C-24 are dealkylated. Exposure of the ciliates to the appropriate sterol sharply curtails the synthesis of the native pentacyclic triterpenoid alcohols, tetrahymanol and diplopterol. An analysis with modified sterols has revealed several additional features that are required for desaturation at C-7,8 and C-22,23 and for inhibition of tetrahymanol biosynthesis. The presence of atrans-17(20)-double bond, which eliminates free rotation at C-20 and fixes C-22 to the right of the nucleus, does not interfere with desaturation, while thecis- or left-handed isomer is not metabolized. Thecis-Δ17(20)-isomer is, however, an effective inhibitor of tetrahymanol biosynthesis, although less so that thetrans-counterpart. When a methyl or hydroxyl group at C-20 protrudes to the front of the molecule in the right-handed conformation, metabolism is reduced or abolished. Shortening (by one C-atom) or lengthening of the sterol side chain has little effect on the ability of the compounds to inhibit tetrahymanol biosynthesis or to support growth, as long as the overall length of the side chain does not exceed seven carbons from C-20. The presence of a 7α-, 7β-, 20α-, 20β-, or a 25-hydroxy group in the cholesterol molecule sharply inhibits desaturation and curtails the effectiveness of the compound as an inhibitor of tetrahymanol biosynthesis. The 7- or 22-keto derivatives seem to act in a fashion similar to the hydroxy derivatives, but these compounds show greater inhibition of growth. 20-Methylcholesterol, however, is a potent inhibitor of synthesis, which suggests that the polarity of the substituent of C-20 is more important than bulk. Many sterols can effectively replace tetrahymanol in the membranes of these ciliates. However, several of the compounds, which inhibit synthesis, appear to be physiologically inappropriate, and poor growth results. An example of the latter class is 20-methylcholesterol. Finally, a class of sterols, represented by 20α-hydroxycholesterol and 7-ketocholesterol, does not severly inhibit tetrahymanol synthesis but leads to growth inhibition and surface abnormalities. These sterols apparently lead to a disordered membrane, even in the presence of tetrahymanol.