IN man the reduction of bilirubin by the intestinal bacteria normally leads to the formation of i-urobilinogen and stercobilinogen, which are excreted partly as such and partly as i-urobilin and laevorotatory stercobilin respectively. Broad-spectrum antibiotic therapy modifies the intestinal flora and unchanged bilirubin is then excreted. Upon with-drawal of this therapy, the alimentary canal again acquires reducing bacteria, and there is temporarily excreted a dextro-rotatory pigment (d-urobilin) which, although urobilinoid in its physical and chemical properties, is not an enantiomorph of stercobilin1. d-Urobilin has been shown to be isomeric with mesobilirubin and mesobiliviolin (C36H42N4O6) and is readily reduced, catalytically or by sodium amalgam, to mesobilirubinogen. The pigment is thus established as a IXα tetrapyrrolic structure and since the spectral properties closely resemble those of i-urobilin and stercobilin the compound must also contain the dipyrryl-methene chromophore characteristic of the urobilins. d-Urobilin is a labile pigment except when pure, and can therefore be obtained only in small quantities; this fact has hitherto precluded its investigation by degradation methods.
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GRAY, C., NICHOLSON, D. Structure of d-Urobilin. Nature 180, 336–337 (1957). https://doi.org/10.1038/180336a0
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