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.

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.

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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Bilirubin (1) bound to human serum albumin undergoes an asymmetric photocyclization to give optically active (2), which exhibits bisignate long-wavelength circular dichroism.