646 PREFERENTIAL FORMATION OF LUMIRUBIN BY GREEN LIGHT

Abstract

Green light is useful in the treatment of neonatal jaundice, and has been reported to be as effective as narrow-spectrum, high-intensity blue light and more effective than standard white light. These reports have been met with skepticism because fluorescent green lights have a spectral output with limited overlap with the absorption band of bilirubin and are less effective than either white or blue lights in producing the configurational isomer of bilirubin (E-bilirubin). Recent studies indicate that the formation and excretion of lumirubin, a structural isomer of bilirubin, is more important than E-bilirubin for pigment elimination in jaundiced infants receiving phototherapy. To assess the effectiveness of green light in producing the structural isomer, I have measured the action spectrum for the formation of lumirubin. I have found that the quantum yield for lumirubin formation (lumirubin production per photon absorbed by bilirubin) is wavelength dependent. The quantum yield was constant for blue light between 430 and 470 nm. Green light at 500 nm had a quantum yield more than twice that of blue light, while 510 nm light had a quantum yield nearly four-fold higher. This unexpected wavelength dependence for lumirubin formation suggests that green light is preferentially absorbed by that half of the bilirubin molecule which undergoes the photochemical reaction to lumirubin. The higher quantum yield for lumirubin formation provides an explanation for the observed clinical efficacy of green fluorescent lights. These results suggest that phototherapy lights with high intensity in the spectral region above 490 and no irradiation in the genotoxic region below 450 nm may prove both safer and more effective than any lamp now in use.