72 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



The Bile Salts. — The bile contains the sodium salts of complex 

 amido-acids called the bile acids. The two acids most frequently 

 found are glycocholic and taurocholic acids. The former is the more 

 abundant in the bile of man and herbivora ; the latter in carnivorous 

 animals, like the dog. The most important difference between the 

 two acids is that taurocholic acid contains sulphur, and glycocholic 

 acid does not. 



Glycocholic acid (C26H43NO6) is by the action of dilute acids and 

 alkalis, and also in the intestine, hydrolysed and split into glycocine 

 or amido-acetic acid and cholalic acid. 



C26H43N06+H20 = CH2.NH2.COOH + C24H4o05 



[glycocholic acid] [glycocine] [cholnlic acid] 



The glycocholate of soda has the formula C26H42NaN06- 

 Taurocholic acid (C26H45NO7S) similarly splits into taurine or 

 amido-ethyl-sulphonic acid and cholalic acid. 



C26H45N07S + H20 = C2H4.NH2.HS03 + C24H4o05 



[taurocholic acid] [taurine] [cholalic acid] 



The taurocholate of soda has the formula C26H44NaN07S. 



The colour reaction called Pettenkofer's reaction, described in the 

 practical exercises at the head of this lesson, is due to the presence 

 of cholalic acid. The sulphuric acid acting on sugar forms a small 

 quantity of a substance called furfuraldehyde, in addition to other 

 products. The furfuraldehyde gives the purple colour with cholalic acid. 



The Bile Pigments. — The two chief bile pigments are bilirubin 

 and biliverdin. Bile which contains chiefly the former (such as dog's 

 bile) is of a golden or orange-yellow colour, while the bile of many 

 herbivora, which contains chiefly biliverdin, is either green or bluish 

 green. Human bile is generally described as containing chiefly bili- 

 rubin, but there have been some cases described in which biliverdin was 

 in excess. The bile pigments show no absorption bands with the spec- 

 troscope ; their origin from the blood pigment has already been stated 

 (p. 70). 



Bilirubin has the formula CigHigNaOg: it is thus an iron-free 

 derivative of haemoglobin. The iron is apparently stored up in the 

 liver cells, perhaps for future use in the manufacture of new haemo- 

 globin. The bile contains only a trace of iron. 



Biliverdin has the formula Ci6H,8N204 (i.e. one atom of oxygen 

 more than in bilirubin) : it may occur as such in bile ; it may be 

 formed by simply exposing red bile to the oxidising action of the 

 atmosphere ; or it may be formed as in Gmelin's test by the more 

 vigorous oxidation produced by fuming nitric acid. 



