Affinity of young rat cerebral slices for bilirubin and some factors influencing its transfer to the slices.

Kinetics of bilirubin transfer to the cerebral slices prepared from 14- to 16-day-old Wistar rats and some factors influencing the transfer were investigated. Within the physiological pH range the rate of transfer was 3 times higher at pH 6.9 than at pH 7.4. The rate at pH 7.2 was a hyperbolic function of unbound bilirubin concentration [the maximum transfer rate (Vmax) = 1.5 mumol/20 min/g slice; the unbound bilirubin concentration to give half the Vmax value (Km) = 200 microM], while at pH 6.9 the saturation curve was biphasic with two sets of kinetic parameters (0.4 and 1.9 mumol/20 min/g slice for Vmax, and 18 and 480 microM for Km, respectively). The kinetics showed that at unbound bilirubin concentration lower than 0.1 mM the Km value at pH 6.9 was about one tenth of that at pH 7.2. The transfer process appeared to be independent of Na+ and was significantly stimulated by Tris+. The rate was reduced at low temperature, and inhibited by urea (competitive inhibition; the concentration to cause 50% inhibition = 100 mM) and sulfhydryl blocking reagents such as p-chloromercuribenzoate and N-ethylmaleimide.