METABOLISM 177 



reserve is drawn upon for the support of the internal activities 

 of the body and as a source of energy for the performance of 

 external work. 



251. Mobilization of reserve fat. In order that the stored 

 fat may be used for the general metabolism of the body it 

 must first be transferred from the adipose tissue cells to the 

 localities where it is needed. Presumably this is accomplished 

 by its reconversion into " soluble fat " and its passage through 

 the walls of the cells into the blood, that is, by a reversal of 

 the process by which it was laid down. Since the transfer of 

 fat through the epithelial cells in resorption is effected by a 

 hydrolytic cleavage (152), one is tempted to imagine a similar 

 reversible enzymatic process in this case. Direct evidence of 

 this is lacking, but apparently such a cleavage takes place some- 

 where at an early stage in the katabolism of the fats, the re- 

 sulting glycerol perhaps serving as a source of dextrose. From 

 that point on the katabolism is , that of the fatty acids. 



252. Oxidation at the p carbon atom. The oxidation of the 

 fatty acids, either saturated or unsaturated, to carbon dioxid 

 and water, like the other katabolic processes already considered, 

 is a step by step process. The researches of Knoop, Embden, 

 Dakin and others 1 have rendered it highly probable, if not cer- 

 tain, that the oxidation, at least in the case of the normal satu- 

 rated acids, begins at the /3 carbon atom (i.e., at the second 

 carbon atom from the COOH group) and results in the splitting 

 off of two carbon atoms at a time. The products are carbon 

 dioxid, water and a fatty acid containing two less carbon 

 atoms than the original one and with which the same process 

 of erosion is repeated. 



If it be true that the fatty acids thus undergo katabolism in 

 the body by stages of two carbon atoms each, and particularly 

 if it may be regarded as probable that they may be built up 

 again in a similar manner from simpler atomic chains, there 

 is afforded a plausible explanation of the rather striking fact 

 that nearly all of these compounds found in the animal body 

 contain an even number of carbon atoms. 



This scheme does not provide for the oxidation of the three 

 lower acids of the series, propionic, acetic and formic, and in 



1 Compare, Dakin, Oxidations and Reductions in the Animal Body, 1912, pp. 

 17-47- 



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