602 VI. OCCURRENCE OF LIPIDS IN THE ANIMAL 



tion. It was found that their degree of saturation could readily be changed 

 by food fats, rapidly when greater unsaturation was involved, and more 

 slowly when the opposite change was brought about. 371 Thus, Sinclair 

 noted 372 that the iodine values for the fatty acids of phospholipids extracted 

 from the whole carcass of rats varied from 104 to 165, depending upon the 

 nature of the dietary fat. However, the ratio of solid to liquid fatty acids in 

 the phospholipids was constant. The differences in unsaturation of the 

 acids are rather to be ascribed to differences in the relative amounts of the 

 several types of unsaturated acids. 



On the other hand, the feeding of a fat containing saturated acids, such 

 as would occur when hydrogenated coconut oil was given, does not alter 

 the unsaturation of the phospholipid fatty acids. 373 Moreover, such satu- 

 rated fats were unable to represent the effect of small amounts of highly 

 unsaturated fats in increasing the iodine number of the tissue phospholipid. 

 Once established, these highly unsaturated fatty acids were shown to be 

 maintained over long periods, despite prolonged fasting or the feeding of a 

 diet rich in saturated fat. In later work, Sinclair 46 showed that elaidic 

 acid may be substituted in the liver phospholipids in the proportion of one- 

 third of the fatty acids. The entrance of this unphysiologic acid into the 

 phospholipids of the liver was rapid, while it occurred only slowly in the 

 muscle phospholipid. On the basis of these results, Sinclair was led to 

 postulate that two types of phospholipids exist in cells, insofar as functional 

 activity is concerned. One class of these compounds, which consists of 

 the more highly unsaturated phospholipids, functions in the essential 

 make-up of the cell. The other type, which is made up of the less unsatu- 

 rated phospholipids, acts as an intermediary product in the metabolism of 

 fat. Terroine 351 suggests that this eclectic point of view may offer the 

 promise of a compromise between the diametrically opposed opinions of 

 Terroine and of Sinclair concerning both the constancy and the qualitative 

 uniformity of the phospholipids. 



{2) The Nature of the Variable Component 



The variable component of fat present in the animal body increases or 

 decreases with an augmentation or decline in the extra calories ingested, as 

 compared with those used. The composition of this deposit fat varies with 

 the nature of the diet, with the sex of the animal, with environment, with 

 the activity of the tissue, and with the site at which the fat is deposited. 



371 R. G. Sinclair, /. Biol. Chem., 95, 393-408 (1932). 



372 R. G. Sinclair, J. Biol. Chem., Ill, 261-273 (1935). 



373 R. G. Sinclair, /. Biol. Chem., Ill, 275-284 (1935). 



