BIOSYNTHESIS OF CHOLESTEROL 389 



b. Cyclopentenophenanthrene Compounds. When cholesterol is syn- 

 thesized in surviving livers from labeled acetate, impurities are always 

 present which have a higher specific activity than the newly- synthesized 

 cholesterol. These compounds, precipitable by digitonin, have been 

 observed within one hour after the injection of carboxyl-labeled C^^-acetate 

 into rats. 2-^ One of the impurities was shown by Schwenk et al.'^'" to be 

 3,o-dihydrox3'-6-keto cholestane; this compound has likewise been de- 

 tected as an impurity in commercial cholesterol. 



The possibihty that A'*-^-cholestenone may be an intermediate in choles- 

 terol s\ni thesis was investigated by Anker and Bloch.--* Earlier tests of 

 Anchel and Schoenheimer*- with deuteriocholesterol were indecisive. 

 Cholestenone is kno\\Ti to be a normal component of the feces of rats and 

 dogs;--^ its conversion to coprostanol (coprosterol) was demonstrated in 

 dogs by Schoenheimer et al?^ Obviously, cholestenone is closely related 

 metabolically to cholesterol. However, Anker and Bloch^^s concluded that, 

 although cholestenone can be converted to cholesterol, it is not an inter- 

 mediate in the total synthesis of cholesterol. It is presumably an inter- 

 mediate in the cholesterol -^ dihydrocholesterol reaction. 



{5) The Source of the Carbons in Biosynthesized Cholesterol 



By the use of doublj^-labeled acetic acid, it has been possible to trace the 

 ultimate disposition of the methyl and carboxyl carbon of the acetate in 

 the synthesis of cholesterol. These atoms were found to be fairly uni- 

 formly distributed in the isooctyl side-chain and in the steroid ring. Little 

 and Bloch^^^ showed that the methyl gi'oup was the source of carbons 18, 19, 

 26, 27, and presumably 17, while carbons 25 and probably 10 were deri\'ed 

 from the carboxyl carbons. Wiiersch, Huang, and Bloch^"^ adduced proof 

 that, in the side-chain, carbons 21, 22, 24-, 26, and 27 were derived from 

 the methyl group, while carbons 20, 23, and 25 arose from the carboxyl car- 

 bon. Cornforth and associates-'^^-^^^'^^" concluded in 1953 that carbon 

 atoms 2, 4, 6, and 10 in the ring originate from the carboxyl carbon of ace- 

 tate, while carbons 1, 3, and 5, in the ring and 18 in the side-chain originate 



226 E. Schwenk and N. T. Werthessen, Arch. Biochem. Biophys., 42, 91-93 (1953). 



2" E. Schwenk, N. T. Werthessen, and H. Ro-senkrantz, Arch. Biochem. Biophys., 37, 

 247-251 (1952). 



22« H. S. Anker and K. Bloch, J. Biol. Chem., 178, 971-976 (1949). 



2" O. Rosenheim and T. A. Webster, Biochem. J., 37, 513-514 (1943). 



"M J. W. Cornforth, G. D. Hunter, and G. Popjdk, Arch. Biochem. Biophys., 42, 481- 

 482(1953). 



