326 IV. WAXES, HIGHER ALCOHOLS, ETC. 



pounds, from which it can be separated only with extreme difficulty. For 

 example, Schonheimer and his collaborators ^^^~^^^ reported that cholesterol 

 prepared from various organs contains 1 to 2% of dihydrocholesterol, as 

 well as traces of ergosterol or of some other highly unsaturated sterol. 

 Provitamin Dg, or 7-dehydrocholesterol, is present in the material isolated 

 from the spinal cord to the extent of 0.1%, while a larger proportion occurs 

 in the sterol separated from skin.^^^ Only after precipitation of the spar- 

 ingly soluble cholesterol dibromide^^*"^^^ and regeneration of cholesterol by 

 zinc dust and acetic acid^^*'**^-^^^ or sodium iodide can a product of high 

 purity be obtained. ^^^ 



Hilditch^^ reports that the liver fat of several species of fishes contains a 

 non-saponifiable fraction which is largely cholesterol. These include the 

 liver fat of the skate {Raia maculata) (0.37%), of the angel-fish (Rhina 

 squatina) (1.5%), of the thresher shark {Alopecias vulpes) (1.8%), and of 

 the spotted dogfish {Scyllium canicula) (2%). 



The relatively high cholesterol value in the mammary gland is probably 

 related to its excretion in milk. Whole milk has been reported to contain 

 0.013%, while powdered whole milk has a cholesterol content of 0.088%.^'^ 

 Nataf and collaborators^^* have found cholesterol to be lowest in the milk 

 of Holstein cows (7.0, 8.2, and 11.1 milligram per cent) and considerably 

 higher in Jersey milk (11.1-15.6 milligram per cent) and in the milk from 

 Guernsey cows (11.8-16.0 milligram per cent). The concentration in but- 

 ter fat is given as 0.185%, w^hile lard is stated to have 0.108% of choles- 

 terol."^ The presence of cholesterol in beef tallow has also been reported. 



The high proportion of cholesterol in skin is of especial interest because it 

 presumably is a precursor of 7-dehydrocholesterol, which is provitamin D?. 

 It is believed that the activation of this provitamin D to vitamin D takes 

 place at or near the surface of the skin when it is exposed to sunlight or to 

 other sources of ultraviolet light. Large amounts of cholesterol are also 

 excreted by the sebaceous glands. Wool grease, which is more commonly 

 called lanolin, is an important source of cholesterol. Separation in a pure 

 form is hampered by the fact that the isocholesterols, which are now known 



151 R. Schonheimer, H. v. Behring, R. Hummel, and L. Schindel, Z. physiol. Chem., 

 192, 73-76 (1930). 



1*^ R. Schonheimer, H. v, Behring, and R. Hummel, Z. physiol. Cheni., 192, 86-93, 

 93-96 (1930). 



15' H. R. Rosenberg, Chemistry and Physiology of the Vitamins, Interscience, New York, 

 1945, pp. 346, 348. 



1" A. Windaus, Ber., 39, 518-523 (1906). 



165 A. Windaus and A. Hauth, Ber., 39, 4378-4384 (1906). 



"« R. J. Anderson, J. Biol. Chem., 71, 407-418 (1927). 



^" L. Ruzicka, H. Briingger, E. Eichenberger, and J. Meyer, flelv. Chi in. Acta, 17, 

 1407-1416 (1934). 



158 R. Schoenheimer, J. Biol. Chem., 110, 461-462 (1935). 



i5» B. Nataf, O. Mickelsen, A. Keys, and W. E. Petersen, J. Nutrition, 36, 495-506 

 (1948). 



