Absence of Dimers and Nature of Iron Binding in Transferrin Solutions

Abstract

A CONTROVERSIAL question in the chemistry of the transferrins is whether the two specific sites of the proteins bind metal ions pairwise or randomly. Equilibrium dialysis studies by Aasa et al.¹, in which care was taken to show that true equilibrium had been achieved, indicated that the stability constants for the sites were virtually identical; this suggested that the binding sites were equivalent and non-interacting. This work was corroborated by studies using the techniques of moving boundary electrophoresis² and isoelectric focusing³. In each of these studies, three species of transferrin were recognized and it was suggested that these represented molecules containing nought, one or two iron atoms. This assignment has been challenged by Wood-worth et al.⁴, who argued that the species with intermediate electrophoretic mobility or intermediate isoelectric point was really a dimer composed of molecules devoid of iron and molecules containing two iron atoms. This alternative interpretation was taken as supporting the earlier contention of Woodworth based on kinetic studies that iron atoms bind to transferrin only pairwise, and that solutions of transferrin less than fully saturated with iron would contain only two separable species of protein molecules, those without any iron and those with two iron atoms per molecule⁵.