Studies of the dipyrrylmethene ("fuscin") pigments. II. The contrasting ratios and significance of the fecal urobilinogen and mesobilifuscin in certain anemias

It has been well established that bilirubin ex-creted in the bile is normally converted to the urobilinogen group in the colon through a series of reductions by the bacterial flora. This subject has been reviewed in recent papers (1-3) and need not be considered in detail here. Maly (4, 5) first suggested the enterogenous (bacterial) for-mation of urobilin and this was first demonstrated in 1893 in the classic study of von Muiller (6) who fed urobilin-free swine bile by tube to an in-dividual having complete biliary obstruction and subsequently recovered urobilin from the feces and urine. However, later attempts to demon-strate urobilinogen formation after tube-feeding of solutions of crystalline bilirubin were generally disappointing; the amount of bilirubin converted to urobilinogen in these studies was 10 per cent or less (7-10). It has also been apparent that measurement of urobilinogen excretion does not consistently reflect the rate of hemoglobin catabo-lism and that discrepancies of considerable mag-nitude are at times noted (1, 2). It has been suggested that the failure to re-cover urobilinogen after tube-feeding of crystal-line bilirubin might be due to conversion of the bilirubin to dipyrrylmethenes such as mesobili-fuscin and, similarly, that discrepancies between urobilinogen excretion and hemoglobin destruc-tion might be reconciled by measurement of meso-bilifuscin excretion (11-13). These concepts, however, were not supported by recent studies (14, 15) which indicated that at least the majority of the naturally occurring fecal dipyrrylmethene or mesobilifuscin is anabolic in origin and reflects pyrrol synthesis on the pathway toward heme, rather than hemoglobin destruction. Nevertheless, *Aided under a contract with the Research and De