In Vitro Kinetic Studies on Gamma Irradiation of Aqueous Bilirubin in the Presence and Absence of Oxygen

Abstract

Bilirubin is associated with positive and negative biological effects. The health effect of bilirubin, in depleting elevated levels of peroxyl radicals in cells, has recently come to the fore. On the other hand, bilirubin is linked to jaundice and the mechanism of bilirubin phototherapy treatment involves isolation of the bilirubin from the subject by means of photoisomerization and conversion to biliverdin. In both cases, the reversion of biliverdin to the parent specimen and its conversion to other undesirable isomers is complex, and therefore, there is a biological need to study such biliverdin isomers exclusively, in the absence of photolytical and unwanted chemical effects. Gamma‐induced (137Cs source) depletion of bilirubin under oxic (in air and O2) and anoxic (N2) conditions was investigated, and led to the exclusive co‐production of the “short‐wavelength” isomer of biliverdin. The depletion of the unconjugated specimen (4.0 × 10−2 mM in 0.05 M aqueous NaOH) was monitored spectrophotometrically, and the results were treated mathematically to facilitate evaluation of the data. The presence of molecular oxygen enhanced the degradation process in comparison to irradiation in air. When the experimental conditions were changed to nitrogen, a marked change in the depletion process was observed. The kinetics of the depletion process provided an insight into the regulation of the procedure. The respective rate constants of the depletion process in O2, air, and N2 were 0.33, 0.25, and 0.19 min−1. Production of biliverdin was controlled by the radiation dose, and in general, growth‐curves were obtained for biliverdin at doses below 100 Gy. Above this dose, no increase in biliverdin concentration (absorbance) was observed under oxic conditions, while increased formation of biliverdin was recorded under anoxic conditions. Our work represents the development of a convenient method for the controlled production of biliverdin for indirect application in biochemical and phototherapy studies.