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Kinetic study of photochemical and thermal conversion of bilirubin IX alpha and its photoproducts.

ABSTRACT: A kinetic study of the photochemical and thermal conversion of photoisomers, especialy peaks 0 and 3 [(EE)-bilirubin IX alpha and (EZ)- and (ZE)-bilirubin IX alpha], under anaerobic conditions, was performed by using reversed-phase high-pressure liquid chromatography. Peaks 0 and 3 are spontaneously, photochemically and thermally converted. Short-term photoirradiation of bilirubin gives a mixture containing not only the geometric isomers (EE)-, (EZ)- and (ZE)-bilirubin IX alpha, but also peak 2, (EZ)-cyclobilirubin IX alpha. On prolonged irradiation, cyclization and 15Z leads to 15E isomerization lead to gradual accumulation of two pairs of photoproducts: (EZ)-cyclobilirubin IX alpha A and B and (EE)-cyclobilirubin IX alpha A and B.

SUBMITTER: Isobe K

PROVIDER: S-EPMC1154147 | biostudies-other | 1983 Mar

REPOSITORIES: biostudies-other

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Kinetic study of the photochemical changes of (ZZ)-bilirubin IX alpha bound to human serum albumin. Demonstration of (EZ)-bilirubin IX alpha as an intermediate in photochemical changes from (ZZ)-bilirubin IX alpha to (EZ)-cyclobilirubin IX alpha.

Project description:The present study was performed to elucidate why the photochemical reaction of (ZZ)-bilirubin bound to human serum albumin is singularly selective, and only one of the two (EZ)- and (ZE)-bilirubins, the (ZE)-isomer, is produced. In a kinetic study of the photochemical reaction in vitro, the sum of the relative rate constants of photochemical transformation of (EZ)-bilirubin into both (EZ)-cyclobilirubin and (ZZ)-bilirubin, with a significant preference for the former, was proved to be considerably larger than that of the transformation of (ZZ)-bilirubin into (EZ)-bilirubin. Therefore only one of the geometrical isomers, namely (ZE)-bilirubin, is apparently formed. It was concluded that (EZ)-bilirubin photochemically undergoes (EZ)-cyclization, i.e. structural photoisomerization, while bound to its high-affinity site on human serum albumin, and is an intermediate in the transformation of (ZZ)-bilirubin into (EZ)-cyclobilirubin.

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High-pressure liquid chromatographic analysis of anaerobic photoproducts of bilirubin-IX alpha in vitro and its comparison with photoproducts in vivo.

Project description:To carry out photochemical experiments under conditions similar to those prevailing for neonatal bilirubin metabolism in jaundice phototherapy, we have studied photoproducts produced by the action of light on a bilirubin--albumin solution and further clarified the relationship between the photoproducts obtained from experiments in vitro and in vivo. (1) An accurate and sensitive separation method by high-pressure liquid chromatography for photoproducts of bilirubin under anaerobic irradiation of visible light is described. (2) There were two main photoproducts obtained from experiments both in vivo and in vitro. (3) Exact correspondence of retention time on high-pressure liquid chromatography, diazo-reactivity, thermal reversion and absorption-spectrum maxima was observed between unknown pigment and photobilirubin-IX alpha from biological fluids, and the comparable peaks 2 and 3 from experiments in vitro. (4) The behaviour of photoproducts in various solutions in the absence of light and O2 is described. (5) A lower affinity of photoproducts, especially unknown pigment, for human serum albumin than with bilirubin-IX alpha for the albumin was demonstrated by the gel-filtration method.

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Comparison of kinetic study of the photochemical changes of (ZZ)-bilirubin IX alpha bound to human serum albumin with that bound to rat serum albumin.

Project description:It has been stated by McDonagh, Palma & Lightner [(1982) J. Am. Chem. Soc. 104, 6867-6871] that complexing of bilirubin with serum albumin has a marked species-dependent influence on bilirubin photoisomerization in vitro and in vivo. Therefore the kinetics for the quantitatively important reaction: (Formula: see text) of the photochemical interconversion between bilirubin and its photoisomers bound to human or rat serum albumin in aqueous solution, assayed by h.p.l.c., was used to elucidate the observed species-dependent difference. The relative rate constants for bilirubin bound to human serum albumin, except for k4, the rate of interconversion from (ZZ)-bilirubin into (EZ)-bilirubin, proved to be considerably larger than those for bilirubin bound to rat serum albumin. In accordance with these rate constants, the formation of photoisomers of bilirubin bound to human serum albumin, except for (EZ)-bilirubin, is very rapid and much greater than that for bilirubin bound to rat serum albumin.

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