704 METABOLISM 



The characteristic feature of cocoanut oil is that its fatty acids are volatile in steam 

 and are saturated. Some of the fatty acids of the liver are volatile in steam, but they 

 are unsaturated. By distillation in steam of the fatty acids obtained by saponification 

 of the liver, it is possible to determine how much of the cocoanut oil has passed to the 

 liver. 



Similar results have been obtained 1 when unsaturated fatty acids, such 

 as those contained in cod-liver oil, are fed. In all these cases the rela- 

 tionship of the liver fat to that of the food is even more evident than 

 that between food fat and depot fat, because in the liver the newly absorbed 

 fat is not diluted by that deposited it may be months previously, as is 

 the case in the connective tissues. 



The question now arises: What happens to the fat during its stay in 

 the liver f An indication of the nature of the change is furnished by 

 observing the iodine value of the fat. This, it will be remembered, in- 

 dicates the degree to which the fatty acid is unsaturated. It does not 

 necessarily indicate the number of unsaturated bonds present in the fatty- 

 acid molecule, for the difference in iodine-absorbing power may depend 

 not on the number of such bonds but on the position in the chain at 

 which a given double bond is inserted. Even with this reservation, how- 

 ever, it is evident that the increase observed in the iodine values shows 

 that the liver has the power of desaturating fat. The advantage of 

 this change depends on the fact that the desaturated fatty acid will 

 be more liable to break up than the saturated fatty acid. In other words, 

 the double linkage will weaken the chain with the consequence that it is 

 liable to fall apart at this place; such at least is the natural interpreta- 

 tion which the chemist would put on the result. It may not, however, 

 be the correct interpretation, for it has been shown that, although un- 

 saturated fatty acids are more susceptible to chemical change in the 

 laboratory than saturated, yet when fed to animals they appear to be 

 more stable than many saturated acids. It may then be wrong to con- 

 clude that the introduction of a double linkage in fat necessarily means 

 the liability of the fatty-acid chain to break at that point. However 

 this may be, it seems likely that one function of the liver consists in 

 introducing double linkages at places in the fatty-acid chain, as a result 

 of which the chain breaks at these places, and the fragments then undergo 

 further oxidation. 



Double linkages may be introduced not only in one place in a fatty- 

 acid chain, but in several. For example, it has been found in the liver 

 of the pig, after oxidizing the fatty acids with permanganate, that oxida- 

 tion products are obtained indicating the existence of unsaturated acid 

 with four double links. Permanganate (in alkaline solution) is used for 

 detecting the position of these double bonds, because, when it is allowed 



