Inter-alpha-trypsin inhibitor is a complex of three different protein species

We found that, in contrast to earlier reports, inter-alpha-trypsin inhibitor of human serum is not a single-chain glycoprotein, but a protein complex. Three different protein components which are encoded on three independent mRNAs could be demonstrated by sequencing of cloned cDNA as well as of peptides obtained from purified inter-alpha-trypsin inhibitor. Accordingly, after fractionation of plasma by ammonium sulfate precipitation, ion-exchange chromatography, zinc-chelate chromatography, and SDS-polyacrylamide gel electrophoresis three proteins can be detected in Western blots using antibodies directed against inter-alpha-trypsin inhibitor.