734 METABOLISM 



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

 the case in the connective tissues. 



Changes in the Fat Deposited in the Liver. An indication of the nature 

 of the change is furnished by observing the iodine value of the fat. This, it 

 will be remembered, indicates the degree to which the fatty acid is unsatu- 

 rated. 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 reserva- 

 tion, however, 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 oxidation products are obtained indicating the ex- 

 istence 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 

 to act on unsaturated fatty acids in the cold, it causes hydroxyl groups to be introduced 

 in the position of the double bonds. When the oxidation is performed at a moderate 

 temperature, the fatty acid falls apart at the hydroxyl groups. A fatty acid with eight 

 hydroxyl groups has been obtained in this way from the liver of the pig. The presence 

 of the hydroxyl groups has been confirmed by finding that an octobromide is obtained 

 by treatment with bromine. An acid of the same formula is said to be present in cod- 

 liver oil. To sum up, we may conclude that there are certain positions, in the chains of 

 carbon atoms which constitute the fatty-acid radicle, where the liver introduces double 

 bonds, and that the weakened radicles then circulate to the tissues, where they break up 

 at those positions. 



Desaturation is probably not the only process by which the liver assists in 

 the metabolism of fat. It may also take part in the 'building of fatty- 

 acid radicles into the complex molecule of lecithin. The process of de- 



