190 III. OXIDATION AND METABOLISM 



ately responsive to any qualitative or quantitative changes in fat metab- 

 olism. 



(a) The Nature of Liver Fats. The liver fats differ from the ordinary 

 ingested fat and from depot fats in having a higher phospholipid content 

 and a greater degree of unsaturation. 



a'. Phospholipids: The relatively high phospholipid content in the 

 liver has led to the hypothesis that lecithin is involved in the intermediary 

 metabohsm of fat. It was suggested that fat is changed into lecithin in 

 order to allow it to pass into the tissue cells, since this phospholipid repre- 

 sents a form of fat which is readily transportable.^^" It w^as also assumed 

 that lecithin is susceptible to oxidation. 



In spite of the high phospholipid content in the liver, the proportion of 

 this component present in the organ is normally fairly constant under a 

 variety of conditions.^^^"®^^ Plowever, Artom^^^ reported that the phos- 

 pholipid content of the liver was increased as much as 35% after the feeding 

 of large doses of fat. On the basis of this effect, Artom''^* considers that 

 the phospholipids represent normal intermediates in fat metabolism. 



Another fact which indicates the importance of phospholipids as inter- 

 mediates in the oxidation of fat in the liver is their relatively high rate of 

 turnover. Thus, Artom, Sarzana, and Segre^^^ reported that, of the 

 various tissues examined, radioactive phosphate was most rapidly in- 

 corporated into the phospholipids of the small intestine and liver. This 

 rate of utihzation by the liver was much higher on a fat diet than it was on 

 a carbohydrate regimen. Fries et al.^^^ confirmed the fact that the feeding 

 of fat increased the rate of turnover of P^- in the liver and intestine. It 

 would thus appear highly probable that phospholipid formation is inti- 

 mately connected with the catabolism of fat by the liver. 



b'. Unsaturation of Liver Lipids: Under ordinary conditions, the 

 liver lipids contain a far larger proportion of unsaturated fatty acids than 

 are found in the lipids of other tissues,^" with the exception of the brain. ^^^ 

 This high misaturation is not confined to the fatty acids in the phospho- 

 lipids, but also applies to those which comprise the neutral fat.^^'^" In 



650 O. Loew, Biol. Zentr., 11, 269-281 (1891). 



6" A. Mayer and G. Schaeffer, /. phijsioi. path, g^n., 16, 325-336 (1914). 



662 W. R. Bloor, J. Biol. Chem., 80, 443-454 (1928). 



6" R. G. Sinclair, /. Biol. Cheyn., 82, 117-136 (1929). 



6" C. Artom, Arch. fisioL, 32, 57-86 (1933). 



655 C. Artom, G. Sarzana, and E. Segre, Arch, intern, physiol, 47, 245-276 (1938). 



656 B. A. Fries, S. Ruben, I. Perlman, and I. L. Chaikoff, /. Biol. Chem., 123, 587-593 

 (1938). 



657 W. F. Bloor and R. H. Snider, J. Biol. Chem., 87, 399-413 (1930). . 



658 R. H. Snider and W. R. Bloor, /. Biol. Chem., 99, 555-573 (1933). 



