II. CHEMISTRY 



135 



system, always at C-5. Examples: cholesterol, sitosterol, stigmasterol, 

 brassicasterol, campesterol, fucosterol, and others (Fig. 2). 



2. The saturated ring type, in which there is no double bond in the ring 

 system. Examples: dihydrocholesterol (cholestanol), dihydrositosterol (si- 

 tostanol), etc. (Fig. 3). 



3. The ergosterol, or provitamin, type, characterized by two double 

 bonds in the ring system, always at C-5 and C-7. Examples: ergosterol 

 (provitamin D2), 7-dehydrocholesterol (provitamin D3), and the incom- 

 pletely identified provitamins D of mollusks (Fig. 4). 



TT p CHCCHs) •CH2'CH2'CH2*CH(CHj)2 



Fro. 1. The sterol ring structure, with a typical side chain at C-17. Rings A, B, C, 

 and D are sometimes designated I, II, III, and IV. 



Cholesterol 



/3-Sitosterol 



Stigmasterol 



Brassicasterol 



Campesterol 



Fucosterol 



R = 



R - 



R = 



R = 



R = 



R = 



— CH(CH3)-CH2-CH2-CH2CH(CH3)2 

 — CH(CH3)-CH2-CH2-CH(C2H6)-CH(CH3)2 

 — CH(CH3)-CH=CH-CH(C2H6)-CH(CH3)2 

 — CH(CH3)-CH=CH-CH(CH3)-CH(CH3)2 

 — CH(CH3)-CH2CH2-CH(CH3)-CH(CH3)2 

 — CH(CH3)-CH2-CH2CH=C(CH3)-CH(CH3)2 



Fig. 2. Examples of cholesterol-type sterols. 



Cholesterol (Fig. 2) is the principal sterol of the higher animals, or 

 vertebrates. It is abundant in nerve tissue, fats, and skin, and present to 

 some extent in all bodily structures. It is usually accompanied by traces of 

 other sterols, which in a few instances have been identified as dihydro- 

 cholesterol (cholestanol) and 7-dehydrocholesterol. 



In the invertebrates cholesterol is partly or wholly replaced by mixtures 

 of other sterols, and in these mixtures 7-dehydrocholesterol and other pro- 

 vitamins D may be present in considerable amounts. The sterols of mollusks 

 are of particular interest in connection with vitamin D, and among these it 

 is to be noted that the sterols of the Pelecypoda (bivalves, such as clams 

 and oysters) differ from those of the Gastropoda (snails). Examples are 



