370 VIII. CHOLESTEROL AND RELATED STEROLS 



is increased after the administration of cholesterol or of cholestenone.''"'"^'' 

 Apparently, the conversion of cholesterol to coprostanol is not a direct one, 

 inasmuch as it involves a change in the steric configuration of the cyclo- 

 pentenophenanthrene nucleus. The hypothesis of Rosenheim and Web- 

 ster^^ and of Anchel and Schoenheimer*- that the pathway of transformation 

 involves cholestenone, coprostanone, and coprostanol has been adequately 

 proved in the dog,^* as well as in man,^- by the use of deuterium-labeled 

 cholesterol and labeled cholesterol intermediates. 



However, dissenting opinions are advanced by Rosenfeld et alP who 

 demonstrated by isotope studies that cholestenone is not a necessary inter- 

 mediate in the hydrogenation of cholesterol. Snog-Kjaer and co-workers** 

 therefore conclude that the conversion of cholesterol to coprostanol in the 

 intestine occurs by direct bacterial hydrogenation by certain anaerobes. 



For a more complete discussion of the changes in sterols in the intestine, 

 the reader is referred to The Lipids, Vol. II, pages 271-274. 



4. Cholesterol in the Blood 



(/) The Nature of Cholesterol in the Blood 



Cholesterol forms the chief component of the unsaponifiable fraction of 

 blood. It is present in the blood both in the form of the free alcohol which 

 is directly precipitable by digitonin, and in the form of the ester, which 

 can be precipitated by digitonin only after hydrolysis. About two-thirds 

 of the plasma cholesterol is normally esterified in the case of man. On the 

 other hand, the cholesterol of human red blood cells occurs largely in the 

 form of the free alcohol. The same is the case with the leucocytes. West 

 and Todd*^ cite the normal range of esterified cholesterol in human serum 

 as 90 to 1 14 mg. %, and that of free cholesterol as 60 to 70 mg. %. Boyd*^ 

 gives the mean value (in mg. %) for cholesterol in the blood plasma of nor- 

 mal women as follows : total, 162; combined, 115; and free, 47. 



" S. Bondzynski and V. Humnicki, Z. physiol. Chem., 22, 396-410 (1896-1897). 

 '8 R. Schoenheimer, D. Rittenberg, and M. Graff, J. Biol. Chem., Ill, 183-192 

 (1935). 



" G. Bischoff, Bidchem. Z., 227, 230-236 (1930). 



8» C. Doree and J. A. Gardner, Proc. Roy. Soc. (London), B 80, 227-239 (1908). 



81 O. Rosenheim and T. A. Webster, Nature, 136, 474 (1935). 



82 M. Anchel and R. Schoenheimer, J. Biol. Chem., 125, 23-31 (1938). 



8' R. S. Rosenfeld, D. K. Fukushima, L. Hellman, and T. F. Gallagher, Federation 

 Proc, IS, 284 (1954); J. Biol. Chem., 211, 301-311 (1954). 



8* A. Snog-Kjaer, I. Prange, and H. Dam, Experientia, 11, 316-317 (1955). 



85 E. S. West and W. R. Todd, Textbook of Biochemistry, Macmillan, New York, 1951, 

 pp. 544, 545. 



86 E. M. Boyd, /. Biol. Chem., 101, 323-336 (1933). 



