CH. XXXV.] BILE SALTS AND PIGMENTS 533 



amino-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, such as the dog. An important difference between the two is 

 that taurocholic acid contains sulphur, and glycocholic acid does not. 

 Glycocholic acid (C 26 H 43 N0 6 ) is by the action of dilute acids and 

 alkalis, and also in the intestine, hydrolysed and split into glycine or 

 amino-acetic acid and cholalic acid. 



C 26 H 43 N0 6 + H 2 - CH 2 .NH 2 .COOH + C 24 H 40 O 5 . 



[Glycocholic acid.] [Glycine.] [Cholalic acid.] 



The glycocholate of soda has the formula C 26 H 42 NaN0 6 . 

 Taurocholic acid (C 20 H 45 N0 7 S) similarly splits into taurine or 

 amino-isethionic acid and cholalic acid. 



C 26 H 45 N0 7 S + H 2 = (CH 2 ),.NH 2 .S0 2 .OH + C 24 H 40 O 6 



[Taurocholic acid.] [Taurine.] [Cholalic acid.] 



The taurocholate of soda has the formula C 26 H 44 NaN0 7 S. 



The colour reaction called Pettenkofer's reaction is due to the 

 presence of cholalic acid. Small quantities of cane sugar and strong 

 sulphuric acid are added to the bile. The sulphuric acid acting on 

 sugar forms a small quantity of a substance called furfuraldehyde, in 

 addition to other products. The furfuraldehyde gives a brilliant 

 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 

 bilirubin ; but this is by no means always so. The bile pigments 

 show no absorption bands with the spectroscope. 



Bilirubin has the formula C 32 H 36 N 4 6 : 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 (C 16 H 18 K 2 4 )^; the value of x 

 is uncertain: 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. 



Gmelin's test consists in a play of colours green, blue, red, and 

 finally yellow, produced by the oxidising action of fuming nitric acid 

 (that is, nitric acid containing nitrous acid in solution). The end or 

 yellow product is called choletelin, C 3 . 2 H 3C N 4 12 . 



Hydrobilirubin. If a solution of bilirubin or biliverdin in dilute 



