Preparation and partial characterization of iron-sulfur, iron-selenium, and iron-tellurium complexes of bovine serum albumin

Abstract

An artificial Fe-S ∗ protein was prepared by the reaction of bovine serum albumin with FeSO 4 and Na 2S or with a synthetic Fe-S ∗-1,4-butanedithiol complex. These improved methods enabled us to characterize the derivatives from serum albumin. The Fe-S ∗ albumin complex has about 20 iron ions and 14 labile sulfur atoms per molecule of the protein, whose absorption spectrum closely resembled that of 2Fe-2S ∗ proteins. Its electron paramagnetic resonance spectrum exhibited signals different from those of ferredoxins. The addition of p-chloromercuriphenylsulfonate quenched the optical absorption in the visible region as well as the electron paramagnetic resonance signals. These properties of the albumin-iron complex are similar to those of iron-sulfur dithiothreitol and mercaptoethanol complexes, suggesting that the albumin-iron complex has one or more protein ligands besides sulfur ligands. Presumably, the oxygen atom of the tyrosine residue, or other hydroxyamino acids participates in the complex formation. In this context, the albumin polypeptide appears to be incapable of forming an iron-sulfur cluster identical to those of ferredoxins. Yet, from the albumin-iron derivative, the extrusion of the iron-sulfur core with benzenethiol provided products similar to those from ferredoxins. The iron-selenium and iron-tellurium derivatives of the bovine serum albumin were prepared and partially characterized by optical absorption and electron paramagnetic resonance spectroscopies. These results imply that both selenium and tellurium can be incorporated into the protein molecule as the respective labile components.