In vitro identification of a soluble protein:geranylgeranyl transferase from rat tissues.

The gamma subunit of mammalian trimeric G proteins has been shown previously to be modified in vivo on a cysteine residue situated at the carboxyl-terminal sequence-Cys-Ala-Ile-Leu-COOH by a 20-carbon prenyl moiety geranylgeranyl (Mumby, S. M., Casey, P.

Inhibition of cholesterol synthesis and cell growth by 24(R,S),25-iminolanosterol and triparanol in cultured rat hepatoma cells.

24(R,S),25-Iminolanosterol (IL) and triparanol added to cultures of rat hepatoma cells, H4-II-C3 (H4), interrupt the conversion of lanosterol to cholesterol and, depending on their concentrations, cause the accumulation in the cells of intermediates in the lanosterol to cholesterol conversion. At 45 microM, both substances cause the accumulation of 5 alpha-cholesta-8(9),24-dien-3 beta-ol (zymosterol), and at the low concentration of 4.5 microM, they cause the accumulation of cholesta-5.

Concerning the role of 24,25-dihydrolanosterol and lanostanol in sterol biosynthesis by cultured cells.

Rat hepatoma cells (H4-II-E-C3) efficiently converted a dietary supplement of [2-3H]24,25-dihydrolanosterol (1) to [3H]cholesterol while [2-3H]lanostanol (4,4,14 alpha-trimethylcholestanol (2) was recovered from the cells without apparent transformation, although it was esterified and induced an accumulation of lanosterol. A comparison of the chromatographic (TLC, GLC and HPLC), spectral (MS and 1H-NMR) and physical properties of 1 and 2 is given for the first time. The inability to detect 2 in nature coupled with our findings that 1 but not 2 is metabolized to cholesterol by H4 cells is interpreted to imply that the biosynthetic inclusion of the delta 8(9)-bond during the cyclization process of squalene-oxide to a tetracyclic product is an evolutionary adaptation selected for because the olefinic linkage is structually important in the subsequent conversion of lanosterol and its stereoisomers, e.

Effects of 2,3-iminosqualene on cultured cells.

2,3-Iminosqualene (ISq) is a powerful inhibitor of squalene oxide:lanosterol cyclase (EC 5.4.99.