136 IV. BILE PIGMENTS 



tained little, if any, of the latter (1713). Heilmeyer (i^i5) and Watson 

 (2981) meanwhile isolated "stercobilin" from urine. Its constitution 

 as tetrahydromesobilene was established by Fischer (Section 6.3.1.). 



In spite of this, the claim that mesobilane and urobilinogen are 

 identical, was nev^er clearly abandoned b}- the Fischer school (cf. 

 Siedel, 2553). The confusion was hidden behind a nomenclature 

 which distinguished between "stercobilin" and "stercobilinogen" on 

 the one hand and urobilin, mesobilene-(b), and urobilinogen, meso- 

 bilane, on the other, although this differentiation was in evident 

 contradiction to the intestinal origin of urinary urobilinogen and 

 urobilin, which was now well known (cf. Chapter XI). 



By the ferric chloride test and by spectroscopy, Lemberg and 

 collaborators (1713; cf. also Baumgartel, 19J^) found that no normal 

 urines and only a few pathological urines contain more than 20% of 

 the total urobilinoids as mesobilene. Watson found that all urines 

 containing abnormal amounts of urobilin were levorotatory, due to 

 the presence of the strongly levorotatory "stercobilin" (2989, p. 2484). 

 He isolated mesobilane as well as "stercobilin" from normal feces. It 

 thus appears that both substances are about equally absorbed from 

 the intestine (2995). The sensitive copper test for mesobilane (for- 

 mation of copper-biliviolin, 777) also indicates the presence of some 

 mesobilane in most urines (8Jf5). Miiller observed as early as 1893 

 that the zinc biliviolin bands appear "nicht ganz selten," if extracts 

 of human feces in ammoniacal zinc salt solution stand exposed to the 

 air (1995). The latter two tests, however, are not quite conclusive 

 since they are given by bilirubin, which may have been present in 

 the urines. 



It has now become clear, then, that both feces and urine contain 

 the two urobilinogens, mesobilane and tetrahydromesobilane and the 

 two urobilins, mesobilene and tetrahydromesobilene (cf. Table VIII), 

 but the tetrahydro compounds usually prevail. A. third urobilinogen 

 should exist, corresponding to the c?-urobilin described below. 



The problem of the formation of the urobilinogens and urobilins 

 from bilirubin will be discussed in Chapter XI. 



6.2. Mesobilane and Mesobilenes 



6.2.1. Structure. Bilanes are the colorless chromogens of the 

 bile pigments (cf. Tables II and VIII). They are very weakly basic 

 compounds, the proton being added to the pyrrole ring with a pK. of 



