Bilirubin as an anti precipitant against copper mediated denaturation of bovine serum albumin: formation of copper-bilirubin complex

BR forms a beautiful 1 : 1 greenish complex with Cu2+, having a characteristic absorption maximum at 343 nm. To our knowledge, this is the first metal complex of bilirubin in aqueous solution so far to be reported. It has also been shown that bilirubin (BR) at a concentration more than protein binding capacity has a definite role as an anti precipitant of bovine serum albumin by excess copper. At a concentration of 1 X 10(-3) moll(-1) of the Cu-BR complex, the colour becomes greenish black. The rate of formation of this Cu-BR complex when BR extracts copper from copper-albumin complex as obtained in our experiment is 34.981mol(-1) s(-1). The Cu-BR complex is stable at a pH ranging from 3.5 to 13.2. and also can scavenge radicals like CCl3OO., e(aq)(-) and OH.. Addition of excess copper sulfate to the solution of this complex causes a greenish black precipitate which can be re dissolved in HCl but insoluble in 99% ethanol. The complex does not give positive test in Gmelin reaction for bile pigments. The fluorescence spectrum of the complex in solution exhibits a peak at 450 nm when excited at 343 nm. The precipitated complex is insoluble in a number of solvents like ether, xylene, benzene, acetone, chloroform-methanol mixture and DMSO. These results show that BR may protect mammals from copper poisoning. (C) 1998 Elsevier Science B.V.