The effects of anions on the kinetics of reductive elimination of iron from monoferrictransferrins by thiols.

The kinetics of reductive elimination of iron from the human serum monoferric transferrins by thioglycollate (TG), 3-mercaptopropionate (MP), cysteine (Cys), cysteamine (Cym) and 2-mercaptoethanol (ME) have been studied at 37 degrees C using bathophenanthroline sulphonate (BPS) as the ferrous ion acceptor. Analysis of the entire course of the reaction was possible only with thioglycollate since the other thiols cause eventual protein precipitation; in these cases, initial rates were used. The rate of iron release is linearly dependent on thiol concentration at low concentrations of reductant (less than approx. 0.2 M) and increases more rapidly with higher concentrations (up to 0.5-0.75 M). The thiols fall into two distinct groups, with TG, MP and Cys reacting at approx. the same rate, which is an order of magnitude faster than the reaction with Cym and ME. The carboxylate functionality present in the first group may be responsible for the faster reaction rate, by competitively weakening the interaction between the protein and synergistic anion. The pH-dependence of the rate of reductive elimination appears to depend on ionizable functionalities on both the protein and reducing agent. The addition of NaCl, NaClO4, NaHCO3 and Na2HPO4 increases the rate of iron release from the monoferric transferrins. The last two have particularly large accelerating effects and, in the case of the N-terminal monoferrictransferrin, gave saturation kinetics, suggesting that the observed effect is due to conformational changes in the protein caused by binding of ions. The role of the Fe-synergistic anion complex in the transferrins as a 'trapped intermediate' is considered.