Urobilinogen, as a Bile Pigment Metabolite, Has an Antioxidant Function

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In these experiments, the antioxidative activity of urobilinogen, a major fecal pigment-related compound and reduced metabolite of bilirubin, was shown. Urobilinogen was synthesized from commercial bilirubin by reduction, and the structure was confirmed with mass spectroscopy and NMR analyses. The product was identified as i-urobilinogen. The DPPH radical scavenging activity of urobilinogen was higher than other antioxidants (α-tocopherol, bilirubin and β-carotene). Moreover, the formation of hydroperoxides of linoleic acid with 2, 2’-azobis (2, 4- dimethylvaleronitrile) in methanol was suppressed by the addition of synthesized urobilinogen. On the other hand, in the NMR spectrum measurement, we found the signal decrease of a proton from C10 methylene. It was shown that this proton was exchanged with a deutrium of CD3OD. This methylene can easily release the hydrogen as proton donor. These results suggest that urobilinogen can inhibit the radical chain reaction by trapping free radicals.