Inhibition of cholesterol synthesis by oxygenated sterols

Sterols derived from cholesterol by introducing a ketone or hydroxyl function in the 6, 7, 15, 20, 22, 24, or 25 positions are known to be potent inhibitors of sterol synthesis in cell cultures. To gain more information regarding structural requirements for inhibitory activity, inhibitory potencies were determined for a series of 18 C₂₇-steroids with various combinations of ketone and hydroxyl functions substituted in positions 3, 4, 5, 6, and 7, or with a single ketone or hydroxyl function in one of these positions. The effects of nuclear double bonds upon inhibitory activity were also examined. A ketone or hydroxyl function in position 3 and a second ketone or hydroxyl function in position 6 or 7 was required for inhibitory activity with two kinds of cell culture. A 3β5α6β-triol was not more inhibitory than a comparable 3β,6β-diol. Cholestance-3β 5α-diol inhibited sterol synthesis in L cells but not in liver cell cultures. The inhibitory activities of 7-oxygenated sterols were not markedly affected by the presence of a double bond in position 4 or 5. Current knowledge of the mechanism through which the oxygenated sterols suppress cholesterol synthesis is reviewed.