Concerning the Proposed Existence of a One-iron–Protein Species of Conalbumin or Siderophilin

Abstract

THE iron-binding proteins siderophilin (transferrin) from human blood plasma and conalbumin, an analogous protein from hen's egg white, are known to bind firmly two ferric ions per protein molecule^1,2. Controversy exists as to whether the two iron-binding sites show positive interaction leading to all-or-none binding of iron^3–5, or show equal and independent behaviour^6–8. One of the strongest arguments for the latter point of view has been the apparent demonstration of an intermediate one-iron–protein species in electrophoretic experiments by moving boundary^7,9 and isoelectric focusing⁸ techniques at low pH. Our failure to observe this intermediate species on polyacrylamide gel electrophoresis at pH 7.5 and the reported failure to observe it at pH 9.2 (ref. 10) led us to investigate further the nature of the proposed “one-iron–one-protein” species.