Bilirubin oxidation by brain mitochondrial membranes is not affected by hyperosmolality

Brain mitochondrial membranes oxidize bilirubin at a rate that may be biologically significant. Hyperosmolality delays clearance of bilirubin from rat brain. We hypothesized that this might be due to a decrease in brain bilirubin metabolism. Rats were anesthetized and infused with 75 mM/kg urea i.v. over 5 min (n = 7) and sacrificed at 1 h; control rats (n = 7) had no treatment. Crude mitochondrial membrane fractions ('P2') were produced by homogenization and differential centrifugation in sucrose. The change in optical density at 440 nM of a 10-micromol/l bilirubin solution was measured after 60 min incubation with mitochondrial membranes. Hyperosmolality did not affect bilirubin oxidation by brain mitochondrial membranes (t = 1.27, p = 0.23 by unpaired t-test). The lack of effect of hyperosmolality on bilirubin oxidation by brain mitochondrial membranes suggests that decreased clearance of bilirubin from brain in hyperosmolality may be related to changes in transfer of bilirubin across the blood-brain barrier.