Prothrombin, albumin and immunoglobulin A form covalent complexes with alpha1-microglobulin in human plasma

Molecules containing the 33-kDa plasma protein alpha1-microglobulin were isolated from human plasma by anti-(alpha1-microglobulin) affinity chromatography. Five major bands could be seen after electrophoretic separation of the alpha1-microglobulin-containing proteins under native conditions. Immunoblotting demonstrated alpha1-microglobulin in all five bands. Two of these have been described previously: free alpha1-microglobulin and alpha1-microglobulin complexed with IgA (IgA x alpha1-microglobulin). The other three bands were identified as prothrombin alpha1-microglobulin, albumin x alpha1-microglobulin and dimeric alpha1-microglobulin. Prothrombin x alpha1-microglobulin were 1:2 and 1:1 complexes which carried approximately 1% of total alpha1-microglobulin, had molecular masses of about 145 kDa and 110 kDa upon SDS/PAGE and dissociated completely to free alpha1-microglobulin and prothrombin (72 kDa) when reducing agents were added, suggesting that the complexes were stabilized by disulfide bonds. The alpha1-microglobulin molecules did not inhibit cleavage of prothrombin by factor Xa and were bound to the peptides which were released upon activation of prothrombin. Albumin x alpha1-microglobulin, corresponding to 7% of total plasma alpha1-microglobulin, was a mixture between 1:1 and 1:2 complexes, with masses upon SDS/PAGE of approximately 100 kDa and 135 kDa, respectively. Both these complexes dissociated only partially to free alpha1-microglobulin and albumin when reducing agents were added. The albumin x alpha1-microglobulin complexes carried a yellow-brown chromophore similar to free alpha1-microglobulin. The complex-binding to alpha1-microglobulin did not block the fatty-acid-binding ability of albumin. The plasma concentrations of albumin x alpha1-microglobulin and prothrombin x alpha1-microglobulin were estimated to 5.2 mg/l and 1.1 mg/l, respectively.