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The Conversion of Cholesterol to ^5,7,22-Cholestatrien-3ß-ol by Tetrahymena pyriformis

From the ¹ From the Departments of Biology and Chemistry, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010

The protozoan Tetrahymena pyriformis was incubated in a growth medium containing added unlabeled cholesterol or 26-¹⁴C-cholesterol with the following consequences: (a) the biosynthesis of tetrahymanol, a pentacyclic triterpenoid alcohol produced in the organism by squalene cyclization, was inhibited, and (b) the cholesterol was accumulated by the cells and converted metabolically to a mixture of other sterols containing ^5,7,22-cholestatrien-3ß-ol(7,22-bisdehydrocholesterol) as the major component, together with 22-dehydrocholesterol and possibly a small amount of 7-dehydrocholesterol. These three sterols, along with unconverted cholesterol, were obtained in the unsaponifiable lipid fraction from the cells, and were separated from one another by silicic acid-silver nitrate column chromatography of the corresponding acetylated derivatives. The 22-dehydrocholesterol and the recovered cholesterol were rigorously identified by comparisons of various chromatographic, spectral, and other properties of the alcohols and their acetates with those of authentic samples. The tentative identification of the 7-dehydrocholesterol was based on chromatographic comparisons of the acetate derivative with authentic material. The structure of the 7,22-bisdehydrocholesterol, a previously unknown compound, was established on the basis of ultraviolet, infrared, nuclear magnetic resonance, and mass spectral data for both the free sterol and its acetate derivative, and also by chemical conversions of the acetate to known compounds. Thus, hydrogenation of the 7,22-bisdehydrocholesteryl acetate with a Raney nickel catalyst gave lathosteryl acetate, and periodate-permanganate oxidation of the triene acetate gave isovaleric acid. It is suggested that the conversion of cholesterol to 7,22-bisdehydrocholesterol in the cells proceeds by way of either 7-dehydrocholesterol or 22-dehydrocholesterol as intermediate.

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