368 VIII. CHOLESTEROL AND RELATED STEROLS 



Wakil/^ who followed the absorption of C^^-labeled ergosterol. Although 

 Bloor^^ stated that ergosterol cannot be absorbed unless it has been con- 

 verted to vitamin D2, other workers^^'®^ have convincingly demonstrated 

 that this sterol can be utilized by laying hens when given in large amounts. 

 Thus, all evidence points to a selective absorption of the sterols in the 

 gastrointestinal tract. Only cholesterol (and to a smaller extent ostreas- 

 terol) can be absorbed by the several species of animals which have been 

 tested. Exceedingly small changes in structure completely change the 

 fate of the sterol insofar as absorption from the gastrointestinal tract is 

 concerned. The difference in metabolism between cholesterol and the 

 phytosterols may consist in the fact that cholesterol can be rapidly esteri- 

 fied when given orally. This can be deduced from the experiments with 

 several sterols, following feeding or subcutaneous injection ;^^ under these 

 conditions, the other sterols cannot be esterified. For a more complete 

 discussion of the absorption of sterols other than cholesterol, see The 

 Lipids, Vol. II, pages 268-271. 



(3) The Transport of Cholesterol and of Other Sterols 



Because of the inability of the common sterols other than cholesterol to 

 be absorbed from the gastrointestinal tract, information concerning the 

 pathway of transfer from the gastrointestinal tract to the blood stream is 

 available only for cholesterol. It was discovered by Mueller^^ as early as 

 1916 that cholesterol is transported via the lymphatics in the case of the 

 dog; moreover, the proportion of cholesterol in ester form in the lymph was 

 found to be the same, irrespective of whether the sterol was given in free 

 or in esterified form. Biggs and co-workers" likewise confirmed the fact 

 that cholesterol is transported by the lymphatic route; moreover, it was 

 found impossible to demonstrate the transfer of any cholesterol by the por- 

 tal system." In more recent studies with C "-labeled cholesterol, Chaikoff 

 and associates'^ were able to account quantitatively for the absorbed choles- 

 terol in the lymph of rats. About 50% of the cholesterol in the lymph was 

 present as the ester.'^-*^ Presumably, esterification takes place somewhere 



«» D. J. Hanahan and S. J. Wakil, Arch. Biochem. Biophys., U, 150-158 (1953). 



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



«5 R. Schonheimer and H. Dam, Z. phijsiol. Chem., 211, 241-245 (1932). 



66 W. Menschick and I. H. Page, Z. phijsiol. Chem., 211, 246-252 (1932). 



" M. W. Biggs, M. Friedman, and S. O. Byers, Proc. Soc. Exptl. Biol. Med., 78, 641- 

 643(1951). 



68 1. L. Chaikoff, B. Bloom, M. D. Siperstein, J. W. Kiyasu, W. O. Reinhardt, W. (1 

 Dauben, and J. F. Eastham, J. Biol. Chem., 194, 407-412 (1952). 



69 J. L. Bollman and E. V. Flock, Am. J. PhtjsioL, 164, 480-485 (1951). 



