Steroids Sterols

Sterols are monohydroxy alcohols with a four-ring core structure or steroid nucleus (Figure 9). Cholesterol is the most abundant sterol in animal tissues. The tetracyclic structure is uniquely compact and rigid. The unesterified molecule has only one polar site, the hydroxyl on the number 3 carbon. When it is esterified to an acyl group, usually oleic acid, the cholesteryl ester is extremely hydrophobic. The free hydroxyl enables the cholesterol molecule to orient in membranes, a major function of

(A)

Figure 9 Cholesterol structural models: (A) conventional, (B) space-filling, and (C) conformational.

Figure 9 Cholesterol structural models: (A) conventional, (B) space-filling, and (C) conformational.

Psychosine

Ceramide

Psychosine

Ceramide

Cerebroside

Neutral ceramide

Figure 8 Structure of a sphingolipid.

^-Sitosterol

Sitostanol

^-Sitosterol

Sitostanol

Campesterol

Figure 10 Plant sterols

Campestanol

Campesterol

Campestanol

OH'

"OH

Figure 11

Cholic acid 3a,7a,12a-Trihydroxy-5,3-Cholanic acid

Bile acid synthesis from cholesterol.

Chenodeoxycholic acid 3a,7a-Dihydroxy-5ß-Cholanic acid cholesterol. In brain and other nervous tissues, free cholesterol is the major component of myelin and renders the myelin sheath impermeable to electron transfer (dielectric).

Phytosterol and campesterol are the major sterols in plant tissues (Figure 10). The plant sterols and their stanol derivatives (saturated at the 5-6 carbons) along with cholestanol are active in regulating cholesterol absorption. All these sterols are consumed in the diet, and some are being added to foods as positive adjuncts to regulation of cholesterol metabolism.

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