Fatty acid enhancement of the quantum yield for the formation of lumirubin from bilirubin bound to human albumin

During phototherapy for neonatal jaundice, bilirubin is converted into a number of different polar photoproducts. Because fatty acids are bound to human albumin in vivo and have been shown to affect the binding of bilirubin to albumin, we examined the effect of various fatty acids on the photochemistry of albumin-bound bilirubin. Fatty acids of carbon chain length of 10 or more were found to increase by as much as 3-fold the quantum yield for formation of lumirubin, an intramolecularly cyclized isomer of bilirubin. The binding of these same fatty acids was found to affect visible absorption and circular dichroic spectra of the bound bilirubin, a finding previously interpreted to be the result of a conformational change in the bilirubin. The increased quantum yield for the formation of lumirubin appears to be the result of an allosterically induced conformational change in bilirubin which is bound to albumin.