Effect of bilirubin and serum albumin on the Na,K-ATPase activity in the synaptosomal membrane

The absorption spectrum of visible light, characteristic of the free bilirubin being in the aqueous medium, with a single maximum at 440 nm and with the shoulder in the region of 410-420 nm is transformed into the spectrum with two maxima in the region of 460 and 500 nm, respectively, when the pigment is bound in vitro by the synaptosomal membrane. There are two types of sites for bilirubin binding in the membrane particles, differing in the values of constants of association (Ka = 0.6 . 10(5) and approximately 2.02 . 10(5) M-1, respectively) and in the values of the maximum binding of bilidiene (5.0 and 7.0 nmoles/mg of membrane proteins, respectively). The binding of bilirubin by the synaptosomal membrane leads to a decrease in the specific activity of the membrane Na+,K+-ATPase. The enzyme activity is further decreasing when suspension of the membrane particles is exposed to the blue light (lambda max = 450-460 nm) in the presence of bilirubin. The addition of the serum albumin into the incubation medium potentiates the inhibition effect of bilirubin, when the suspension of membrane particles is lighted in the presence of bilirubin. The alkalization of the medium up to pH 7.8 (from pH 7.2) removes this potentiation effect of the addition of serum albumin.