Quantitative analysis of IgA1 binding protein prepared from human serum by hypoglycosylated IgA1/Sepharose affinity chromatography

The binding protein to a hypoglycosylated IgA1/Sepharose (IgA1-BP) could be prepared from human sera. IgG was a major component in the IgA1-BP. A Protein A column was used to remove the IgG; however, about half of the IgA1-BP was passed from the column [Biochem. Biophys. Res. Commun., 264 (1999) 424]. Quantitative analysis of the passed fraction (PAP) by laser nepherometry indicated that it was composed of a fairly large amount of IgA, IgM and complement C3 besides IgG. The relative content of IgG:IgA:IgM:C3:C4 was 25:10:41:22:2 in the PAP fraction. Meanwhile, the Protein A bound-fraction was essentially composed of IgG (78%) and IgM (19%). The total amount of IgA1-BP was not different between the sera from IgA nephropathy patients and other nephropathy patients. With respect to the IgA content in the IgA1-BP from IgA nephropathy patients, it was significantly higher than that from other nephropathy patients. It was found that the IgA1-BP from some IgA nephropathy patients contained a few micrograms of aberrant IgA per ml of serum. Thus, the obtained results suggested the preferential deposition of the self-aggregated IgA composed of hypoglycosylated IgA1 and co-deposition of IgG, IgM and C3 in the glomeruli in an IgA nephropathy patient.