Involvement of Fe(III) in the formation of immunoglobulin G-enriched protein aggregates in human plasma

A small fraction of the proteins present in human plasma can be found as circulating protein aggregates. Such aggregates are formed by prone to aggregation proteins and different stimuli promote the aggregation process. Fe(III) is a redox active metal ion which also actively interacts with proteins. The aim of this work is to identify the prone to aggregation plasma proteins in presence of Fe(III) in order to outline potential targets of these circulating protein aggregates. Here we show that Fe(III) induces the formation of protein aggregates from human plasma proteins. A concentration of 100 μM Fe(III) aggregates roughly 5 % of the total plasma protein assayed. When assayed by SDS-PAGE/silver-staining, a rather homogeneous aggregate can be observed with one major protein with a molecular weight matching that of immunoglobulin G (IgG) (150k Da). Additionally, the band corresponding to albumin (66 kDa) which is the main plasma protein was absent. The identity of IgG within the aggregate and albumin depletion was corroborated by liquid chromatography-mass spectrometry. Additionally, some other proteins could be identified within the aggregate such as fibrinogen, fibronectin and Apo-B. Then, the identity of the IgG and depletion of albumin was corroborated by Western blot. It should be noted that aggregated IgGs are strong activators of inflammatory pathways involving neutrophil oxidative burst, complement cascade activation and platelet release of active amines. Therefore, the existence of a potential link between the formation of Fe(III)-induced protein aggregates and inflammation should be further explored.