Potential role of conjugated bilirubin and copper in the metabolism of lipid peroxides in bile.

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Abstract
Conjugated bilirubin and copper ions at their physiological concentrations in bile may play an important role in hydroperoxide and other detoxification. Conjugated bilirubin may also be an important chain-breaking antioxidant preventing lipid peroxidation. Bilirubin ditaurine (BR-DT), a water-soluble model compound of conjugated bilirubin, completely prevents the peroxyl radical-induced oxidation of phosphatidylcholine in either multilamellar liposomes or micelles. This antioxidant activity is associated with the bilirubin moiety of BR-DT, since taurine alone is inefficient in scavenging peroxyl radicals. The number of peroxyl radicals trapped per molecule of BR-DT is 1.9, compared to 4.7 trapped per molecule of biliverdin, the water-soluble physiological precursor of bilirubin. Peroxyl radical-induced oxidation of BR-DT results in a spectral shift in maximal absorbance toward shorter wavelengths; biliverdin is not formed as a major oxidation product. BR-DT, but neither taurine nor biliverdin, greatly accelerates the cupric ion-catalyzed decomposition of linoleic acid hydroperoxide. In the presence of ferric ion, BR-DT shows no lipid hydroperoxide-degrading activity. Addition of cupric ion to BR-DT results in formation of a complex with spectral features similar to that of a biliverdin-cupric ion complex, indicating that BR-DT and cupric ion undergo redox reactions.