The Conversion of N16-labeled Mesobilirubinogen to Stercobilinogen by Fecal Bacteria*

Under normal circumstances, two members of the urobilinogen group have been identified in human feces and urine, both of which are determined quantitatively in the routine urobilinogen procedure (1). Of these, stercobilinogen predominates, while mesobilirubinogen occurs only in small amounts except during excessive hemolysis, when it has been present in sufficient quantity to permit crystallization from urine (2, 3) and feces.’ Von Miiller (4) and McMaster and Elman (5) initiated a series of studies from which it has been generally concluded that these compounds are formed in the intestinal tract by the bacterial reduction of bilirubin. Earlier experiments in this laboratory (6) indicated that mesobilirubinogen is aetually an intermediary in the formation of stercobilinogen. Despite this evidence, Baumgartel has claimed that mesobilirubinogen is solely of hepatic or biliary origin (7). It is his view that stercobilinogen is formed in the intestine by the bacterial reduction of bilirubin, but that mesobilirubinogen is not an intermediary in this process. This concept has been receiving increasing attention (8-11). One of its principal supports, according to Baumgartel, is the failure, in his hands, of cultures of fecal bacteria to convert mesobilirubinogen to stercobilinogen. Satisfactory evidence of the inability of fecal organisms to effect this conversion would admittedly make untenable the scheme of bile pigment metabolism which is generally held. For this reason, we have reinvestigated the problem with both ordinary and N15-labeled mesobilirubinogen.