Artificial Substrates in Squalene and Sterol Biosynthesis

Trans-[1-3H2]3-methylpent-2-enyl pyrophosphate and [1-14C]isopentenyl pyrophosphate were incubated with 10,000 xg supernatants of rat liver homogenates. Analyses of the unsaponifiable material extracted from the incubations by gas-liquid radiochromatography and mass spectrometry revealed the synthesis not only of [14C]squalene but also of [3H, 14C]1-methylsqualene and of [3H,14C]1,24-dimethylsqualene. The sterol fraction of the unsaponifiable extracts contained two components labelled with 3H and 14C, the 3H:14C ratios of which were identical with the isotope ratios of the 1-methylsqualenes and which had retention volumes in gas-liquid chromatography expected of 27-methylcholesterol and of 27-methyllanosterol. The sterol fractions did not contain components that might have been derived from the cyclization of 1,24-dimethylsqualene. It is suggested that 1-methylsqualene is epoxidized enzymically only at its isopropylidene end, but not at its isobutylidene end.