598 VI. OCCURRENCE OF LIPIDS IN THE ANIMAL 



exclusively of neutral fats, the composition of which depends to a consider- 

 able extent upon the nature and proportion of dietary fat. It may also be 

 related to the level of carbohydrate in the diet, since this foodstuff may 

 serve as a source of much of the storage fat. The quantity of the variable 

 component will naturally depend upon the level of alimentation. 



On the other hand, the constant component of the fat present in an ani- 

 mal, which constitutes as much as 23% of the dry matter in the mouse and 

 25% in the chicken, consists of phospholipids, cerebrosides, and other essen- 

 tial fatty components such as cholesterol, as well as of some neutral fat. It 

 is believed that these lipids serve as essential elements of the protoplasm. 



(1) The Nature of the Constant Component 



Terroine and Belin 352 were of the opinion that the constant component 

 was related both qualitatively and quantitatively to the phospholipids. 

 It was relatively constant, in spite of all physiologic variations. These 

 authors have shown that the ratio, total fatty acids: lipid phosphorus, ap- 

 proximates the value for lecithin. They believe that the lecithin type of 

 phospholipid is the only one present to any considerable degree in this 

 fraction. According to Terroine and Belin, 352 the nature of the food is 

 without influence on the fatty acid composition of the constant component 

 of a wide variety of tissues and organisms. It was also indicated that the 

 iodine number of the phospholipid fatty acids in the lung, kidney, liver, and 

 muscle of the same species was within a relatively narrow range. The un- 

 saturation of the fatty acids was therefore believed to constitute a second 

 index of the constant component of the fatty tissues which was characteris- 

 tic of the species. Thus, the constant component for any species was 

 characterized not only by the uniformity in total quantity but also by the 

 similarity in qualitative composition as expressed in the degree of unsatura- 

 tion of the component fatty acids. 



However, the studies by MacLachlan and co-workers 353 force one to re- 

 vise the concept that phospholipids are all equally important as components 

 of the constant element of fat. Although the total quantity of phospho- 

 lipid in the liver of the mouse is decreased by about 50% after four days of 

 fasting, the size of the liver is likewise reduced to about one-half of what it 

 was previous to fasting. Thus, the concentration of phospholipids remains 

 fairly constant in the liver during inanition. This led Hodge, MacLachlan 



362 E. F. Terroine and H. Belin, Bull. soc. chim. bioi, 9, 12-48 (1927). 

 353 P. L. MacLachlan, H. C. Hodge, W. R. Bloor, E. A. Welch, F. L. Truax, and J. D. 

 Taylor, /. Biol. Chem., 148, 473-490 (1942). 



