5 i8 TEXT-BOOK OF PHYSIOLOGY 



The Metabolism of the Nudeo -proteins. The tissue cells of animal and 

 many vegetable foods contain nuclei into the composition of which nucleo- 

 proteins enter. In the metabolism of the nucleo-proteins it is assumed on 

 the basis of their reaction with hydrolyzing agents that they undergo a 

 cleavage into a protein and nuclein. The nuclein subsequently gives rise 

 to nucleic acid which, under the influence of an enzyme, is separated into 

 two bases, guanin and adenin; these bases under the action of two enzymes, 

 guanase and adenase, are combined with water, deaminized and transformed 

 into xanthin and hypoxanthin. The oxidation of these latter compounds 

 gives rise to uric acid. (See page 18 and 461.) 



THE METABOLISM OF THE FAT 



It will be recalled that during the digestive process the neutral fats are 

 gradually reduced by the action of the enzyme lipase of the pancreatic juice 

 to corresponding fat acids and glycerin; that subsequently the fat acids com- 

 bine with alkalis to form soaps, after which both soaps and glycerin are 

 absorbed. After absorption and during their passage through the epithelial 

 cells covering the villi, the soap and glycerin are synthesized into fat globules 

 and deposited in the cell material. Subsequently the fat granules enter 

 the lymph- vessels of the mesentery and by which they are discharged into 

 the thoracic duct and finally into the blood at the junction of the subclavian 

 and internal jugular veins. After a meal rich in fat, the amount of fat in 

 the blood may be so great as to impart to it a distinctly pink color. In the 

 course of several hours the fat disappears. As to the change it undergoes 

 before it passes across the capillary wall there is much obscurity. It is 

 stated by Hanriot, that a lipase in blood again hydrolizes the fat to fat acids 

 and glycerin and in consequence of their solubility, pass out into the lymph- 

 spaces. By ultra-microscopic methods of illumination the fat may be ob- 

 served as "blood dust," in vigorous Brownian movement (Bayliss). It is 

 possible that under this form they pass into the lymph-spaces. As to the 

 final disposition of the fat the general belief is, that when it is consumed 

 in normal amounts and under physiologic conditions it promptly undergoes 

 oxidation in the tissue cells yielding carbon dioxid and water with the libera- 

 tion of heat. 



The intermediate stages through which fat passes have not been wholly 

 determined. Nevertheless it is a well-founded supposition that the first 

 stage is a cleavage of the neutral fats to fat acids stearic, palmitic and 

 oleic, and glycerin. 



These fat acids are the higher members of the fat acids and are character- 

 ized by a high molecular weight. Before they can undergo complete oxida- 

 tion they must be reduced to lower acids of the fat acid series, acids of a low 

 molecular weight. 



This is accomplished presumably by successive cleavages and oxidations. 

 Thus stearic acid which consists of 18 carbon groups is successively reduced 

 by oxidation of the carbon atom in the beta position until the four carbon 

 stage is reached when the resulting acid is known as butyric acid, e.g., 

 CH 3 .CH 2 .CH 2 .COOH. Upon oxidation of the beta carbon atom there is 

 formed /3-oxybutyric acid, viz.: CH 3 .CHOH.CH 2 .COOH. 



The succeeding oxidation results in the removal of the two hydrogen 

 atoms attached to the beta carbon atom and results in the formation of 



