Interaction of bilirubin with brain capillaries and its toxicity

The interaction of bilirubin with isolated brain capillaries, and the effect of bilirubin on the uptake of 2-deoxyglucose by the capillaries were investigated with 1-month-old Sprague-Dawley rats. The binding of bilirubin to the brain capillaries was observed only at a molar ratio of bilirubin to bovine serum albumin higher than 1.0. An absorption spectrum with a microspectrophotometer of the bilirubin-capillary complex showed a broad absorption maximum from 425 to 440 nm with a shoulder near 490 nm, but no shoulder was observed in the case of the bilirubin emulsion. The bilirubin binding activity was dependent on pH and temperature of the medium, but was not affected by sulfhydryl blocking agents such as p-chloromercuribenzoate and N-ethylmaleimide. Bilirubin saturation kinetics gave an apparent Km for bilirubin of 61.7 microM. Release of bilirubin from the brain capillaries to the medium was observed at 37 degrees C but not at 4 degrees C. The uptake of 2-deoxyglucose by the isolated brain capillaries was inhibited by bilirubin in a noncompetitive manner, giving an apparent Ki for the pigment of 137 microM. These results suggest that bilirubin may be responsible for the decreased 2-deoxyglucose uptake in the brain capillaries by disturbing the membrane structure of the capillary endothelial cells.