Biliverdin and bilirubin are reducing species and hence potential antioxidants formed by the action of heme oxygenase and biliverdin reductase. Indeed, there is increasing evidence for the suggestion that a beneficial role of the potentially toxic bilirubin may be to act as a powerful chain-breaking antioxidant in biological systems, and that bilirubin may contribute to the cellular and tissue protection seen with increased heme oxygenase. This article reviews the in vitro antioxidant activities of the two bile pigments with emphasis on the different physiological forms of bilirubin and types of oxidants, and discusses these properties in light of the presence and reactivity other nonproteinaceous antioxidants. Antioxid. Redox Signal. 6, 841–849.

Antioxidant activities of bile pigments.

Publisher Summary This chapter discusses the antioxidant activities of bile pigments—biliverdin and bilirubin. Biliverdin and bilirubin are potent scavengers of singlet oxygen. The methods described in the chapter apply to the measurement of the peroxyl radical scavenging activity of biliverdin and the various biological forms of bilirubin. Most of the methods are applicable to testing the antioxidant activity of other compounds. The method is based on the thermal decomposition of an azo compound under aerobic conditions whereby peroxyl radicals are produced at a known and constant rate. In the presence of phospholipids containing polyunsaturated fatty acids (LH), the peroxyl radical will abstract a hydrogen atom to give rise to a lipid radical, thus initiating the chain reaction of lipid peroxidation. Because the peroxyl radical-mediated oxidation of polyunsaturated fatty acids initially results in quantitative formation of the corresponding lipid hydroperoxides, the extent of oxidation can be followed simply by measuring the accumulation of hydroperoxides. Any compound possessing peroxyl radical scavenging activity will compete with LH for the lipid peroxyl radical, resulting in inhibition of the chain reaction and hence inhibition of formation of lipid hydroperoxides. The results show that both biliverdin and conjugated bilirubin efficiently scavenge hypochlorous acid and act synergistically with vitamin E in protecting lipid membranes from peroxidation initiated within the lipid phase, this add further credit to the notion that the two bile pigments function as important physiological antioxidants.

[31] Antioxidant activities of bile pigments: Biliverdin and bilirubin

6 – Bile Pigments: Bilatrienes and 5,15-Biladienes

Recent functional studies have suggested that, in addition to simple diffusion, carrier-mediated transport may play an important role in foetal unconjugated bilirubin (UCB) uptake by the placenta. We have investigated the role of organic anion-transporting polypeptides (OATPs) in UCB transport by the placenta-maternal liver tandem. RNA was obtained from human liver (hL), human placenta (hPl) at term, and purified (> 80%) cytokeratin-7-positive mononucleated human trophoblast cells (hTCs). By analytical reverse transcription (RT)-PCR, agarose gel electrophoresis separation and sequencing, the mRNA of OATP-A ( SLC21A3 ) and OATP-8 ( SLC21A8 ) was identified in hL, hPl and hTCs, whereas that of OATP-C ( SLC21A6 ) was detectable only in hL. Real-time quantitative RT-PCR revealed that in hL the abundance of mRNA was OATP-8 > OATP-C >> OATP-A, whereas in hPl and hTCs this was OATP-8 >> OATP-A >> OATP-C. Expression levels for these OATPs were hL >> hTCs > hPl. Injection of mRNA of OATP-A, OATP-C or OATP-8 or RNA from hL, hPl or hTCs into Xenopus laevis oocytes conferred on them the ability to take up [(3)H]17 beta-D-glucuronosyl oestradiol ([(3)H]E(2)17 beta G) and [(3)H]UCB, although in the case of OATP-A mRNA, the induced uptake of [(3)H]UCB was very low. Cis -inhibition of [(3)H]E(2)17 beta G and [(3)H]UCB uptake by both unlabelled E(2)17 beta G and UCB was found in all cases. The affinity and efficiency of [(3)H]UCB transport was OATP-C > OATP-8. Kinetic parameters for [(3)H]UCB uptake induced by RNA from hTCs resembled most closely those of OATP-8. In conclusion, our results suggest that OATP-8 may play a major role in the carrier-mediated uptake of foetal UCB by the placental trophoblast, whereas both OATP-8 and OATP-C may substantially contribute to UCB uptake by adult hepatocytes.

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Role of organic anion-transporting polypeptides, OATP-A, OATP-C and OATP-8, in the human placenta-maternal liver tandem excretory pathway for foetal bilirubin.

Structure and binding of unconjugated bilirubin: relevance for physiological and pathophysiological function.

Determinations of the low equilibrium concentrations of non-albumin-bound unconjugated bilirubin in serum by a kinetic method have been done on normal umbilical cord blood serum, on serum from exchange transfusions of icteric newborns, and on normal adult serum. The concentrations are about 10−9 M in cord serum and in adult serum, and about 10−5 M in the icteric infant sera. The latter shows little correlation between total unconjugated bilirubin and non-bound bilirubin, indicating that there is a considerable variation in the binding of bilirubin to albumin.

O. Fedders

Papers

Determination of non-albumin-bound bilirubin in human serum.