104 IV. BILE PIGMENTS 



hardly explain the astonishing degree of ignorance of this field of 

 biochemistry shown by some textbook writers. The various classes 

 of bile pigments are usually distinguished by their color, e.g., bilirubins, 

 biliverdins, biliviolins, but this nomenclature has not been consist- 

 ently used {e.g., "urobilins"), and it has recently become clear that 

 there are more such classes than can be readily distinguished by 

 their color. 



It is highly desirable that the nomenclature of the side chains be 

 adjusted to coincide with that of the porphyrin series. We should 

 therefore speak of proto and meso compounds. As is customary in 

 the hematin series, the prefix "proto" may be omitted {e.g., "bili- 

 rubin" instead of "protobilirubin"). Inconsistencies introduced by 

 the vagaries of historical development can unfortunately no longer 

 be removed entirely, but priority claims should not stand in the way 

 of a rational nomenclature. Thus, urobilin contains meso side chains, 

 while porphobilin (c/. below) is probably a urourobilin, a "urobilin" 

 with uro side chains. 



The term "bilin" combined with a prefix should only be used to 

 mean a bile pigment from a certain biological source {e.g., phycobilin, 

 pterobilin, helioporobilin, urobilin, stercobilin). The term "glauco- 

 bilin" for mesobiliverdin is a misnomer and should be abandoned. 

 The same holds for urobilin or stercobilin, if these terms are meant 

 to connote a definite chemical structure. At least two urobilins 

 exist in the urine and the same two occur in the feces. The terms 

 stercobilin (Watson) and urobilin should, therefore, not be used to 

 characterize any one of them as a chemical entity, and the same 

 holds for urobilinogen and stercobilinogen. Siedel's term "urobilin 

 IXa" is particularly unfortunate, since the other urobilin ("sterco- 

 bilin") has also the side chain arrangement IXa, and the term "natural 

 urobilin" used by Fischer is definitely misleading.* Copromesobili- 

 violin for a mesobiliviolinoid pigment from feces is another misnomer 

 which has been abandoned. 



* Nothinj? ('ould show better the confusion to which this nomenclature has led 

 than the following statement of Siedel (2557, p. 114): "Wiihrend sich nun sowohl 

 Heilmeyer und Krebs, wie C. J. Watson, flir die Identitat des Sterkobilins mit dam 

 Urobilin aussprachen, entdeckten H. Fischer, Halbach, and Stern als einen entschei- 

 denden Unterschied die optische Aktivitiit des Sterkobilins." In fact the first-named 

 authors had correctly established the identity of the major urobilinoitl constituent 

 (Watson's "stercobiliti") of urine and feces, while the difference in optical activity is 

 between this and the second urobilinoid constituent found occasionally in both urine 

 and feces, the "urobilin IXa" of Siedel. 



