NORMAL FAT METABOLISM 203 



been advanced to explain the way in which the material passes across the 

 vessel walls into the tissues. One of the earliest was that there is the 

 same process of hydrolysis and resynthesis as takes place in the passage of 

 the intestinal wall, which postulates the presence of lipases in the neighbor- 

 hood of where the transfer takes place. In this connection much confusion 

 has resulted from the failure to distinguish between "esterases" enzymes 

 which can hydrolyze simple esters such as ethyl butyrate and also, though 

 more slowly, glycerids of the lower fatty acids, as for example tributyrin, 

 but cannot hydrolyze ordinary fats (or, at least, only very slowly), and 

 true lipases such as are found in the pancreatic secretion, which split fats 

 readily; and still further uncertainty has been caused by the failure to 

 exclude cells or portions of cells from the extracts used for testing. Es- 

 terases appear to be quite widespread in the blood and tissues, although 

 generally in small amounts and of slight activity, while lipases in sig- 

 nificant amounts appear to be confined to the pancreas. Even in the 

 mammary gland and the fat depots where the exchange of fat would 

 presumably be most active no significant amount of lipase can be demon- 

 strated. So that the primary requisite for hydrolysis and resynthesis, an 

 adequate supply of lipase at the tissue cell wall is missing. On the other 

 hand, esterases which are capable of splitting lecithin are found to be 

 quite widely distributed (Thiele, 1912-13) and, for reasons which will 

 appear later in the discussion, are believed to be of importance in fat 

 metabolism. 



Coincident with or immediately following the rise of fat in the blood 

 during fat absorption certain changes have been noted in the other blood 

 lipoids which appear to be of importance in fat metabolism. A consid- 

 erable increase of lecithin is noted by all workers. A similar increase of 

 cholesterol is found by some but not by others, which may be explained by 

 the fact that it apparently comes later. It is becoming more and more 

 evident that these three substances fat, cholesterol and lecithin are 

 closely connected in fat metabolism, and when one is increased the others 

 are very generally also found to be similarly high. The period during 

 which fat is abnormally high in the blood during fat absorption (about 

 eight hours) is apparently long enough to produce increases of lecithin, 

 which follow quickly the increases in fat, but may not be long enough to 

 bring about increases of cholesterol which take place later and more slowly. 

 The close relation of lecithin and cholesterol to fat would indicate that 

 these are stages in metabolism through which the fats may or must pass 

 before they are utilized, a supposition which is supported in the case of 

 lecithin by the close similarity in composition and in the case of cholesterol 

 by the constant relation in the blood serum between cholesterol and its 

 fatty acid esters. 



The blood corpuscles appear to take a considerable part in the changes 

 in the blood lipoids during alimentary lipemia. The old observation of 



