Blue light converts bilirubin in the skin of jaundiced rats to metastable geometric isomers that are transported in blood and excreted in bile. The same reaction probably occurs in jaundiced babies exposed to light, particularly during treatment with phototherapy. Excretion of unisomerized bilirubin is prevented by intramolecular hydrogen bonding, and the pigment has to be metabolized to more polar derivatives to be excreted efficiently.

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Anders als bei der Bindung von (Z,Z)‐ Bilirubin IXa an tierischen Serum‐Albumin (SA) weist die an menschliches SA gebundene Titelverbindung bei ihrer Photoisomerisierung nur eines der beiden möglichen Diastereomeren auf.

Amorphous isomerically pure biliverdin IX alpha is readily prepared in more than 70% yield by dehydrogenation of bilirubin with 2,3-dichloro-5,6-dicyanobenzoquinone in dimethyl sulphoxide under carefully controlled conditions. Crystalline biliverdin IX alpha and amorphous [14C]biliverdin can be obtained similarly in more than 40+ yield. The pure crystalline pigment was characterized by elemental analysis, methylation, chemical and enzymic reduction to bilirubin, i.r.- and u.v.-visible-absorption spectroscopy, n.m.r. spectroscopy and field-desorption mass spectrometry, and its solubility was determined. Under certain conditions, dehydrogenation, gave biliverdin contaminated with III alpha and XIII alpha isomers as a result of disproporationation of bilirubin. Formation of non-IX alpha isomers depends on the concentrations of the reagents and the order in which they are mixed, and occurs under neutral anaerobic conditions. Free-radical reactions probably are responsible, suggesting that the first step in the deydrogenation of bilirubin with 2,3-dichloro-5,6-dicyanobenzoquinone in dimethyl sulphoxide is formation of a bilirubin cation radical, rather than hydride ion abstraction.

Abstract. In hepatobiliary disease bilirubin becomes bound covalently to serum albumin, producing a nondissociable bile pigment-protein complex (biliprotein). To elucidate the mechanism of biliprotein formation we studied the bile pigment composition of blood from animals with experimental cholestasis and carried out comparative studies on the rate of biliprotein formation in vivo and in vitro during incubation of bilirubin glucuronides with albumin. Bile duct ligation in the rat and guinea pig led to rapid accumulation in the circulation of bilirubin, heterogeneous bilirubin esters of glucuronic acid, and a biliprotein that migrated along with albumin on high performance liquid chromatography. When the obstruction was removed, biliprotein remained longer in the circulation than did the other bile pigment species. Biliprotein and heterogeneous bilirubin esters of glucuronic acid were not formed in bile duct-ligated homozygous Gunn rats but they were formed when bilirubin glucuronides were incubated with Sprague-Dawley rat serum or human serum albumin at 370C in vitro. Bilirubin glucuronide rearrangement in vitro was accompanied by nonenzymic hydrolysis. We conclude that the formation of biliprotein in vivo is probably nonenzymic and suggest that mammalian biliprotein is formed by acyl migration of bilirubin from a bilirubin-glucuronic acid ester to a nucleophilic site on albumin.

Biliverdin was reduced to bilirubin in pregnant and foetal guinea pigs, and the 100000 g supernatant from homogenates of foetal liver, placenta and maternal liver showed high biliverdin reductase activity. The placental transport of unconjugated bilirubin and biliverdin was compared by injecting unlabelled and radiolabelled pigments into the foetal or maternal circulation and analysing blood collected from the opposite side of the placenta. Injected bilirubin crossed the placenta from foetus to mother and vice versa, but injected biliverdin did not appear to cross without prior reduction to bilirubin. The guinea-pig placenta is apparently more permeable to bilirubin than biliverdin. Reduction of biliverdin to bilirubin in the foetus may, therefore, be essential for efficient elimination of haem catabolites from the foetus in placental mammals.

Tin(1V)-protoporphyrin chloride, a proposed chemotherapeutic agent for prevention of neonatal hyperbilirubinemia, is a potent photosensitizer of bilirubin destruction in vitro at visible wavelengths (410, 540, 580 nm). We speculate that the drug may have phototoxic effects in vivo if it accumulates in light-accessible tissues.

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