340 



ANIMAL METABOLISM 



in the usual way to pyruvic acid, which in turn forms acetic acid or a 

 related C2 substance. The long chain fatty acids are then most prob- 

 ably produced by uniting a number of these C2 units. This plan 

 accounts for the fact that nearly all the natural fatty acids contain 

 an even number of carbon atoms. 



Furthermore, it has been shown by the isotope tracer technique that 

 acetic acid does form fatty acids in the animal body. Acetic acid labeled 

 with deuterium in the methyl group and C^^ in the — COOH was given 

 to mice and rats, and the body fats examined after a few days. Both 

 deuterium and C^^ were present in the fatty acids, and in the same amounts 

 relative to each other as in the acetic acid fed. The isotopes were present 

 in all parts of the fatty acid molecules. Other animals were fed deu- 

 terium oxide ("heavy water") in place of ordinary water, and the body 

 fats were found to have taken up the deuterium. These facts are all 

 consistent with the idea that the fatty acids are synthesized by con- 

 densation of Co fragments, followed by reduction with hydrogen derived 

 from water in the body tissues. 



Thiamine is required for fat synthesis, possibly because it is a part 

 of cocarboxylase which is required for the oxidative decarboxylation of 

 pyruvic acid to form acetic acid (reaction 16, Fig. 13-4). Recently it 

 has been found that another vitamin, namely biotin, is involved in the 

 synthesis of oleic acid, particularly in microorganisms. 



Steroid Synthesis. Acetic acid has also been found to serve as a 

 metabolic precursor of cholesterol in the animal body. At least half, 

 and probably more, of the carbon and hydrogen atoms in cholesterol 

 are derived from this source. Several other substances such as ethyl 

 alcohol, leucine, and butyric acid can also take part in cholesterol syn- 

 thesis, but probably only because they are first converted into acetic 

 acid. 



Other important animal steroids are known to be formed, in turn, from 

 cholesterol. Thus the transformation of cholesterol into cholic acid and 

 pregnanediol has been demonstrated with isotopic compounds. 



Acetylation of Amines. When amines, e.g., sulfanilamide, not normally 

 present in the body are given to animals, they often are converted at 

 least partially into acetyl derivatives, which are excreted. This repre- 

 sents a bodily mechanism for throwing off foreign and possibly toxic 

 materials. As a rule, the acetylated products are less toxic than the 

 original amines. 



Acetylation also occurs in the case of normal tissue constituents {e.g., 

 amino acids, choline) and is, in fact, a very common metabolic reaction. 

 It has been amply demonstrated that the acetyl groups come from acetic 

 acid. Acetylcholine, produced by acetylation of choline, is an essential 

 substance for nerve functioning. 



The participation of acetic acid in the metabolic production of por- 

 phyrins and uric acid is discussed under protein metabolism (p. 351). 



