NATURAL WAXES 323 



nucleus. Cliolesterol and a numl)er of closely related compoiuul.s have at 

 l(>a.st one unsaturated linkage in ring B of the cyclic part of the molecule. 

 Ergosterol and other provitamins D have two unsaturated linkages in ring 

 B. 



Many sterols are readily precipitable by the saponin, digitonin, with 

 which they form an insoluble precipitate. However, no digitonide results 

 when this test is applied to a sterol ester, as a free hydroxyl group is a 

 necessary requirement for the reaction. Lanolin and other sterol esters 

 can easily be separated from the readily precipitable free sterols by the 

 application of this method. 



Sterols show a marked tendency to separate in the form of mixed crystals 

 with closely related compounds. In some cases it is almost impossible to 

 remove the contaminant completely, even by repeated recrystallizations. 

 It has recently been found that unsaturated sterols are usually mixed with a 

 small amount of the corresponding dihydro compound, i*^-^*'' 



(f/) Occurrence and Properties of Individual Zoosterols, a'. Cholesterol: 

 Cholesterol is by far the oldest recognized and also the most important 

 member of the sterol group. It belongs to the class of zoosterols. This 

 alcohol is widely distributed in the animal kingdom but it is completely 

 absent from the plant world. It is the sterol which is almost exclusively 

 {)resent in warm-blooded animals, while closely related forms may replace 

 it in the lower animals and hisects. 



(!') Distribution of Cholesterol: Cholesterol is present in most animal 

 cells. Highest concentrations occur in nervous tissue, liver, and in fat de- 

 posits. The white matter of the brain contains 14% of cholesterol on the 

 diy weight basis, while the grey matter has a content of 6% of this alcohol.*^ 

 Fieser^^^ indicates that the human brain may contain as much as 17% of 

 cholesterol in the dry substance. A sufficiently high proportion of choles- 

 terol occurs in the spinal cord of cattle, so that this tissue is widely used 

 commercially as a source. Other tissues have the following amounts of 

 cholesterol (in per cent) based on dry weight^^": kidney 1.6, spleen 1.5, 

 skin 1.3, liver 0.93, mammary gland 0.70, whole blood 0.65, heart muscle 

 0.65, smooth muscle 0.55, diaphragm 0.34, and skeletal muscle 0.25. Bile 

 also contains a considerable amount of cholesterol; this secretion may be- 

 come supersaturated with this alcohol, with the result that gallstones are 

 formed. The adrenal cortex is known to be a site where cholesterol occurs 

 in relatively high concentration. It is considered to function in this organ 

 as the precursor of the adrenocortical honnones. 



Cholesterol is frequently associated with small amounts of related com- 



>« R. J. Anderson, F. P. Nabenhauer, and R. L. Shriner, /. Biol. Chem., 71, 389-390 

 (1926-1927). 



i« F. W. Hey] and O. F. Swoap, /. Am. Chem. Soc, 52, 3688-3690 (1930). 

 '*" W. R. Bloor, Biochemistry of the Fatty Acids, Reinhold, New York, 1943. 



