The steric requirements for the metabolism of sterols by Tetrahymena pyriformis.

Cultures of Tetrahymena pyriformis were incubated with a series of sterols which differed from one another in the steric character of their side chains. (E)-1X20)-Dehydrocholesterol in which C-22 is fixed to the right (bans with respect to C-13, “right-handed”) in the usual view of the molecule was converted to its A5,7,17(2”’ and A5v7,17(20)22 derivatives; however, the organism did not significantly metabolize the “left-handed” (Z)-17(20)-dehydrocholesterol in which C-22 is c&-oriented with respect to C-13. We interpret this to imply a stereochemical requirement for a righthanded side chain, i.e. a side chain with C-22 on the right side. Since both cholesterol and (E)-20(22)-dehydrocholesterol, in which the side chains are not fixed rotationally about the 17(20) bond, underwent dehydrogenation, the conformers which bind to protein presumably are the righthanded ones. A second stereochemical criterion for metabolism was found to be the absence of bulk on the front face of C-20. This derives from the observation that four additional sterols with a substituent at C-20 either were not dehydrogenated or were dehydrogenated only to a limited extent. In 20-isocholesterol, 20/3-hydroxycholesterol, and 20-methylcholesterol a methyl group projects to the front in the right-handed conformation, and none of these sterols was metabolized to a significant degree. 20a-Hydroxycholesterol presents an hydroxyl group instead of a methyl group to the front in the right-handed conformation. It also was not dehydrogenated in the side chain. A small amount of the ‘I-dehydro derivative was obtained, but the quantity was less than in the case of sterols lacking bulk greater than a hydrogen atom in the same steric position. The ability of the organism to metabolize the trigonal and therefore planar (E)-A17(20’and (E&A Z0(20)-sterols proves that tetrahedral character at C-20 is not necessary and that no bulk in the perpendicular plane is required.