550 XI. HEMOGLOBIN CATABOLISM, I 



absorption of bilirubin injected into isolated intestinal loops, Watson {S989, 

 p. 2481) raised the objection that pure, not "natural," bilirubin had been 

 injected; this objection is hardly justified. Watson's own experiments {2979) 

 cannot be considered as evidence in favor of the enterohepatic circulation 

 of bilirubin. 



8.4.3. Excretion of Bilirubin in Urine. According to Rabino- 

 witch {2199) and Naumann {2011) normal urine contains small 

 amounts of bilirubin (about 0.3 mg. per 100 ml.) which can be demon- 

 strated by adsorption to talc and application of the Fouchet test to 

 the adsorbate (c/. Chapter IV, Section 9.1.). With {3108) found 

 urine to contain usually one quarter to one half of the bilirubin con- 

 centration of the plasma, and occasionally the same concentration. 

 The "threshold" is therefore no true threshold, but depends on the 

 sensitivity of the method used for testing the urine for bilirubin. 

 According to Pollock {2160) and Watson (2991a), increase of bilirubin 

 in the urine can be noted in the pre-icteric stage of infective hepatitis 

 earlier than increase of serum bilirubin. 



9. END PRODUCTS OF HEMOGLOBIN CATABOLISM 



9.1. Transformation of Bilirubin to Urobilin 

 in the Animal Body 



9.1.1. Introduction. We have shown in Chapter IV, Section 6.1., 

 that both urobilin and stercobilin (or the corresponding leuco com- 

 pounds, urobilinogen and stercobilinogen) actually consist of mix- 

 tures of tetrahydromesobilene-(b) and mesobilene-(b) (or the corre- 

 sponding mesobilanes), while urobilin and stercobilin are probably 

 identical in composition. The recognition of the complex nature is of 

 comparatively recent date, and has so far received little attention in 

 physiological investigations. Urobilins and urobilinogens are readily 

 intraconvertible and physiologically only the sum of the two is of 

 interest. We shall therefore for most purposes in this section simply 

 speak of urobilin, meaning the sum total of all urobilins and uro- 

 bilinogens, whether in urine or feces. Since we now know that 

 urinary urobilin is derived from fecal urobilin, the use of different 

 names, at least for physiological purposes, is illogical. 



The conversion of bilirubin into tetrahydromesobilane, the sub- 

 stance usually predominating in urobilinogen (Chapter IV, Section 

 6.1.),^ consists in a reduction, eight hydrogen atoms being added to 

 the tetrapyrrolic system and four to the vinyl side chains of bili- 



