The presence of acyl-CoA: cholesterol acyltransferase in Tetrahymena pyriformis W.

1. The esterification of cholesterol was studied in Tetrahymena pyriformis an organism which does not synthesize sterols nor are sterols required for growth. 2.

Rotational isomerism about the 17(20)-bond of steroids and euphoids as shown by the crystal structures of euphol and tirucallol.

The influence of configuration at C-20 on rotation about the 17(20)-bond in steroids and euphoids was examined by x-ray crystallographic studies of the C-20 epimers euphol and tirucallol. The H atom on C-20 was in back next to C-18 in the crystal structures of both of the compounds, and C-22 was found to be cis-oriented ("left-handed") to C-13 in euphol and trans-oriented to it ("right-handed") in tirucallol. The results, which are consistent with the known left-handed crystal structure of 24(25)-dihydroeuphol and right-handed crystal structure of cholesterol and other natural sterols, lend further credence to the earlier suggestion that rotational isomerism at the 17(20)-bond can arise in C-20 epimers and that there is preference for an arrangement with the 20-H atom adjacent to C-18.

A comparison of the biological properties of androst-5-en-3 beta-ol, a series of (20R)-n-alkylpregn-5-en-3 beta-ols and 21-isopentylcholesterol with those of cholesterol.

The delta 5-sterol, androst-5-en-3 beta-ol, which has no side chain at C-17, did not permit molting of the insect Heliothis zea, growth of either the protozoan Tetrahymena pyriformis, or the yeast Saccharomyces cerevisiae adapted to anaerobic conditions, nor was the sterol esterified by a mammalian microsomal ACAT preparation. However, the sterol did form a liposome with egg lecithin and, when fed to mice, did inhibit hepatic cholesterol synthesis. 21-Isopentylcholesterol also formed a liposome but neither supported the growth of the yeast nor was metabolized by the protozoan.

Steric effects at C-20 and C-24 on the metabolism of sterols by Tetrahymena pyriformis.

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.

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.

Effect of sterol replacement in vivo on the fatty acid composition of Tetrahymena.

The addition of ergosterol to cultures of Tetrahymena pyriformis results in (a) the accumulation of the sterol by the cells; (b) the inhibition of the synthesis of the pentacyclic triterpenoid alcohol, tetrahymanol; (c) the replacement of tetrahymanol by ergosterol in the ciliate membranes. The dry weight and lipid content of sterol-supplemented ciliates did not differ from the controls. Examination of the lipid classes revealed no change in composition except for a higher content of ergosterol in supplemented cells than tetrahymanol in control cultures.

Desmosterol as the major sterol in L-cell mouse fibroblasts grown in sterol-free culture medium.

The principal sterol synthesized by L-cell mouse fibroblasts is desmosterol. Cholesterol was not detected in these cells when they were grown in a sterol-free culture medium. These findings indicate that, in cells, cholesterol can be replaced by desmosterol.