Journal of Lipid Research, Vol 22, 770-777, Copyright © 1981 by Lipid Research, Inc.
Steric effects at C-20 and C-24 on the metabolism of sterols by Tetrahymena pyriformis
WR Nes, JM Joseph, JR Landrey, S Behzadan and RL Conner
Cultures of Tetrahymena pyriformis were incubated with various sterols and
the extent of dehydrogenation at C-7 and C-22 was determined. The sterols
incubated were desmosterol, 22-dehydrodesmosterol, 24- methyldesmosterol,
24 alpha-methylcholesterol (campesterol), 24- methylene-cholesterol,
isohalosterol (26,27-bisnorcampesterol, also known as
24,24-dimethylchol-5-en-e beta-ol, a naturally occurring C26- sterol), and
20-isohalosterol. 20-Isohalosterol was not metabolized, while products with
delta 7- and delta 22-bonds were formed from isohalosterol and all of the
other sterols studied. This confirms an earlier conclusion, based on
results with 20-isocholesterol and cholesterol, that inversion of the
configuration from 20(R) to 20(S) completely prevents metabolism both in
the nucleus and the side chain. On the other hand, changes in the
electronics or stereochemistry at C- 24 had a direct affect only on
metabolism in the side chain. The presence of a methyl group at C-24
reduced the yield of metabolites with a delta 22-bond relative to those
with a delta 7-bond producing an accumulation of 7-dehydro metabolite. A
double bond at position-24 counteracted this steric effect, presumably by
enhancing the rate of dehydrogenation, and a delta 24(28)-bond was more
effect than was a delta 24(25)-bond.
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