The effect of carbon monoxide on the nature of the accumulated 4,4-dimethyl sterol precursors of cholesterol during its biosynthesis from [2-14C]mevalonic acid in vitro

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Abstract
Cholesterol biosynthesis was studied in rat liver subcellular fractions incubated with dl-[2-14C]mevalonic acid under gas phases consisting of either N2+O2 (90:10) or CO+O2 (90:10). CO inhibits cholesterol biosynthesis from [2-14C]mevalonic acid and results in a large accumulation of radioactive 4,4-dimethyl sterols. Separation of the components of the 4,4-dimethyl sterol fraction showed that lanosterol and dihydrolanosterol are the major components that accumulate during cholesterol biosynthesis in an atmosphere containing CO, whereas 14-demethyl-lanosterol and 14-demethyldihydrolanosterol are the major components of the much less intensely radioactive 4,4-dimethyl sterol fraction isolated from incubations with N2+O2 as the gas phase. The identities of lanosterol, dihydrolanosterol and 14-demethyldihydrolanosterol were confirmed by both radiochemical and physicochemical methods, including g.l.c. and combined g.l.c.–mass spectrometry. CO therefore results in a qualitative as well as a quantitative difference in the 4,4-dimethyl sterol fraction which arises during cholesterol biosynthesis from mevalonic acid. The specific radioactivity of the [14C]lanosterol biosynthesized in the presence of CO was lower than that of its companion, [14C]dihydrolanosterol. The relative amounts of 4,4-dimethyl-Δ24-sterols and 4,4-dimethyl-24,25-dihydrosterols present in each type of incubation suggest that enzymic reduction of the sterol side chain occurs predominantly at a stage after that of lanosterol.