The effects of branching, oxygen, and chain length in the side chain of sterols on their metabolism by Tetrahymena pyriformis.

The ciliated protozoan Tetrahymena pyriformis was incubated with delta 5-sterols which has side chains varying in degree of branching, polarity, and length. Branching at neither end of the side chain was obligatory for metabolism, although removal of C-21 did have a small quantitative effect. Both 21-norcholesterol and sterols lacking a terminal gem-dimethyl group, e.g. 26 (or 27)-norcholesterol, underwent conversion to the corresponding delta 5,7,22-trienols. However, replacement of C-21 with oxygen (20 xi-hydroxy- and 20-keto-21-norcholesterol) was very deleterious. In particular, introduction of a delta 22-bond was abolished, and dehydrogenation in Ring B was strongly reduced. The overall length of the side chain which permitted maximal metabolism corresponded closely to that in cholesterol and other natural sterols. Maximal metabolism was observed only with sterols bearing side chains which had 5 or 6 carbon atoms (other than C-21) attached to C-20. The extent of metabolism fell gradually to zero as the length of the chain was increased or decreased. No metabolism at all occurred with side chains of no carbon atoms (pregn-5-en-3 beta-ol) or as many as 12 carbon atoms (20(R)-n-dodecylpregn-5-en-3 beta-ol) on C-20 other than C-21. Dehydrogenation in the side chain was more sensitive to chain length than was dehydrogenation in Ring B. The data presented here reinforce the view that the enzymes involved in sterol metabolism are actually directed toward binding with sterols. Furthermore, since the sterols become components of the ciliary membrane of this protozoan, the observed structural requirements for metabolism presumably reflect the structural requirements for membrane architecture; and the evidence presented then suggests that the natural length of the sterol side chain is governed by the sterol's membranous function.