Reactivity of pyrrole pigments, part 13: Identification of the reaction product generated from bile pigments by the superoxide anion

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The UV/Vis spectra of the conjugated bases (NH deprotonation) of biliverdin IX α (BV), mesobiliverdin IX α (MBV), biliverdin IX α dimethyl ester (BV -DME) and mesobiliverdin IX α dimethyl ester (MBV - DME) are shown. They resemble those obtained for the reaction products of these biliverdins with superoxide anion ( $$o_2^{\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{ - } } $$ ). These results confirm that the bile pigments react with $$o_2^{\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{ - } } $$ giving the lactam NH deprotonated conjugated bases and inducing $$o_2^{\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{ - } } $$ dismutation. The spectrometric titrations ofBV, MBV and their dimehyl esters show that the lactam NH of the vinyl substituted biliverdins is more acidic (ΔpK a ℞0.5). The spectra of the lactam NH bisdeprotonated conjugated bases of the bilatrienes-abc studied (BV 4− andMBV 4−) can be obtained inDMSO/H2O/OH− systems of high basicity function (H−℞23). Because of the low oxidation potentials ofBV 3− and of the corresponding trianion of bilirubin IX α (studied by voltammetry) an alternative metabolic degradative pathway is suggested for bilirubin, involving the interaction in lipophilic media with $$o_2^{\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\cdot}$}}{ - } } $$ and oxidation of the conjugated base generated by NH deprotonation.