GENERAL STRUCTURE AND COMPOSITION 13 



The Purin Bodies, of which Uric Add (C 6 H 4 N 4 O^ is the most 

 familiar example, are derived chiefly, if not wholly, from the 

 decomposition of nucleo proteins and are therefore interesting as 

 being the end products of the vital activities of the cell nuclei. 



Pigments. The most important of these which occur in the 

 Body are: 



Hemochromogen, an iron-containing pigment which in com- 

 bination with the histon ylobin forms hemoglobin, the red coloring 

 matter of the blood. When hcmochromogen is in the presence of 

 oxygen it combines with it to form hematin. 



Bilirubin and biliverdin are the bile pigments and give to bile its 

 color. Bilirubin is yellow and biliverdin green. The former 

 usually predominates in the bile of man and the carnivora, making 

 such bile yellow; the latter is the dominant color in the bile of 

 herbivorous animals, which is green. They are closely related 

 chemically and are derived from the decomposition of hemoglobin. 



Urobilin is formed in the intestine as the result of the putrefac- 

 tion there of the bile pigments. It is absorbed thence into the 

 blood and excreted by the kidneys, and imparts to the urine its 

 characteristic yellow color. 



Enzyms are a group of substances which seem to be allied in 

 chemical composition to the true proteins, but it is so difficult to 

 be sure of the purity of any specimen that their composition is still 

 in doubt. The enzyms have the power, even when present in very 

 small quantity, of bringing about extensive changes in other sub- 

 stances, and they are not themselves necessarily used up or de- 

 stroyed in the process. Many enzyms of great physiological im- 

 portance exist in the digestive fluids and play a part in fitting food 

 for absorption from the alimentary canal. For example, pepsin 

 found in the gastric juice converts, under suitable conditions, such 

 complex proteins as albumins into simpler peptones; ptyalin, found 

 in the saliva, converts starch into sugar. We shall have occasion 

 later to study a number of enzyms more in detail in connection 

 with their physiological uses. A characteristic property of all 

 enzyms is their susceptibility to heat; a temperature of 60 C. 

 suffices to destroy them completely. 



Non-Nitrogenous Organic Compounds. These may be con- 

 veniently grouped as hydrocarbons or fatty bodies; carbohydrates 

 or amyloids; and certain non-nitrogenous acids. 



