Prothrombin, albumin and immunoglobulin A form covalent complexes with alpha(1)-microglobulin in human plasma

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