18 



Samuel Gurin and Roscoe O. Brady 



chain of cholesterol. The ratio of CH3/COOH groups incor- 

 porated into the nucleus was calculated to be 1 • 1 while the 

 corresponding value for the side chain was 1-67. 



Carboxyl-labelled short chain fatty acids are also utilized 

 by liver slices for the biosynthesis of cholesterol (Table I). 

 This presumably occurs by preliminary fragmentation into 

 two-carbon units since there is abundant evidence that fatty 

 acids are converted to acetoacetate by way of two-carbon 



Table I 



Biosynthesis of Cholesterol from Short Chain Fatty Acids 



fragments. Evidence has also been obtained suggesting that 

 short chain fatty acids are to a considerable extent degraded 

 into two-carbon units prior to their incorporation into long 

 chain fatty acids (Brady and Gurin, 1950a). 



We have confirmed the finding that the methyl groups of 

 acetate contribute 1 • 3 times as much isotope to cholesterol 

 as do the carboxyl groups (Table II). It will be observed that 

 acetaldehyde is somewhat more efficiently utilized by liver 

 slices than is acetate. We have also found this to be true for 

 the biosynthesis of long chain fatty acids by surviving liver 

 slices (Brady and Gurin, 1951). 



Under similar conditions, three acetoacetate preparations 

 labelled with ^^C in the carboxyl, carbonyl, and methyl - 

 methylene positions respectively, were found to be incor- 

 porated into cholesterol (Table III). In each case aliquots of 

 the same liver were incubated with suitably labelled acetate 

 as controls. Approximately three times as much acetate is 



