Ring Modified Sterols

The ring structure of cholesterol cannot be metabolized to C02 and HfeO in humans. Rather, the intact sterol nucleus is eliminated from the body by conversion to bile acids and bile salts, which are excreted in the feces, and by secretion of cholesterol into the bile, which transports it to the intestine for elimination. Some of the cholesterol in the intestine is modified by bacteria before excretion. The primary compounds made are the isomers coprostanol and cholestanol, which are reduced derivatives of cholesterol. Together with cholesterol, these compounds make up the bulk of (neutral fecal sterols. 

Two molecules of famesyl pyrophosphate are joined head-to-head to form squalene, a triterpene, in the first dedicated step towards sterol biosynthesis (Fig. 8.4). Squalene is then converted to 2,3-oxidosqualene, which next can be cyclized to the 30 carbon, 4-ring structure cycloartenol by the enzyme cycloartenol synthase (EC 5.4.99.8). Cycloartenol can be further modified by reactions such as desaturation or demethylation to form the common sterol backbones such as... 

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