The effect of pH on the binding of sodium aurothiosulphate to human serum albumin. A possible binding mechanism

The effect of pH on the binding of aurothiosulphate to human serum albumin was studied in unbuffered solutions at 37 degrees and ionic strength 0.15-0.16 M. In the investigated pH range, 6.3-8.4, the effect of pH on the high affinity association constant K1 was very different from that on the lower affinity constants K2-K4. K1 was virtually constant except for a two-fold decrease in the narrow pH range 7.5-7.9, which was explained as a H+ induced local conformation change in the environment of site 1. Contrary to this, K2-K4 decreased monotonically with increasing pH, which could be entirely accounted for by a change in electrostatic interaction. A conceivable binding mechanism consistent with the results might be: that gold binds as Au+ to the high affinity binding site by exchanging a H+ and that this site might be the free sulphydryl group in cysteine or the terminal alpha-amino group; and that gold binds as Au(S2O3)3-(2) to the lower affinity binding sites which might be the protonated basic side chain group, i.e. epsilon-amino groups.