Complexes of albumin and α 1-antitrypsin with Fc-fragment of IgA monomer are disulfide-bound to penultimate C-terminal cysteine in the Cα3-domain

Abstract

Six IgA myeloma sera, containing albumin (ALB)—IgA and α 1-antitrypsin (α 1-AT)—IgA complexes (CXs), were gel-filtered on Ultrogel AcA22. The CXs were eluted between monomeric and dimeric IgA. The CX-containing fractions were digested with three IgA-proteases: ALB and α 1-AT remained bound to Fcα-, but not to Fabα-fragments. Short (< 2 h) peptic digestion entirely released free ALB and α 1-AT from the CXs, indicating that these proteins are linked to the third constant domain of the α-chain. Identical results were obtained with anti-ALB immunosorbent-purified ALB—IgA CXs. Reduction-alkylation of ALB or α 1-AT bound to IgA or Fcα confirmed disulfide binding. The data support the binding of ALB and α 1-AT to the same penultimate C-terminal cysteine of the α-chain as that which binds the J-chain.