472 



ANIMAL BIOCHEMISTRY 



ihe small intestine. Bacteria of the intestinal tract then carry out a 

 further reduction to mesobilirubinogen. Some of the latter material 

 is excreted as such; most is probably reduced further to urobilinogen, 

 which is partly reabsorbed, transferred to the liver by the plasma, 

 and again eliminated in the bile. A part of the urobilinogen (also 

 called stercobilinogen) is oxidized in the intestinal tract to urobilin 

 (stercobilin) and is responsible for the brown color of feces. Small 

 amounts of the urobilinogen recycled through the liver are excreted 

 via the urine, where oxidation in light and air also leads to urobilin. 

 Thus hemoglobin is degraded and eliminated from the animal body. 



CH, 



H- 



HOH 



H 



C2H5 



CHo 



-CH, 



COOH COOH 

 I I 



(CH^). (CH2)2 CH 



H 



-CHo 



CH, 



-CH, 



C2H5 

 -H 



HOH 



H 



iiviibilinogen 



H 



COOH COOH 

 I I 



CH3 C2H5 CH3 (CH2)2 (CH2)2 CH3 CH3 CsHr, 



HOH 



HOH 



iirnbilin 



Perhaps other porphyrin-proteins are handled in a similar way, but 

 it is not yet possible to trace them. 



Nucleoproteins and Nucleic Acids 



Animals make use of the components of nucleic acids and nucleo- 

 proteins in their diets. They also synthesize a supply of their own, 

 so that these important materials need not occur in the food supply. 

 On the other hand, the occurrence of nucleoproteins and perhaps 

 nucleic acids in all living materials renders it advantageous to use 

 them in metabolism. 



Ingested nucleoproteins are split by the gastric fluids into nucleic 

 acids and proteins. The latter are then digested as usual and the 

 amino acids utilized as such or metabolized as outlined above. The 

 nucleic acitls pass into the small intestine, where they are probably 

 hydrolyzed by pancreatic and intestinal nucleases into nucleotides. 

 Nucleotidases and phosphatases then yield nucleosides and inorganic 



