Structural relationship between alpha 1-microglobulin from man, guinea-pig, rat and rabbit

Rabbit alpha 1-microglobulin was purified from the urine of sodium-chromate-treated animals by the use of gel chromatography on Sephadex G-100, affinity chromatography on concanavalin-A--Sepharose and ion-exchange chromatography on DEAE-Sephadex. Rabbit alpha 1-microglobulin had a molecular mass of 25.6 kDa on SDS/polyacrylamide gel electrophoresis. Alpha 1-microglobulin has previously been purified from the urine of humans, guinea-pigs and rats by similar methods, and the molecular masses of the four homologues were compared by SDS/polyacrylamide gel electrophoresis and gel chromatography in a denaturing medium. By these two methods the human homologue was 6 kDa and 3 kDa larger, respectively, than the other three proteins. Endoglycosidase F digestion of alpha 1-microglobulin, followed by SDS/polyacrylamide gel electrophoresis, revealed three protein bands in the human alpha 1-microglobulin sample, and only two bands in guinea-pig, rat and rabbit alpha 1-microglobulin, with a gap between each band of 2.6--2.9 kDa. The amino-terminal amino acid sequences of the four homologues were determined and between 72% and 81% homology was seen. The five amino-terminal amino acids present in the other species were missing in guinea-pig alpha 1-microglobulin. Our results indicate that human alpha 1-microglobulin is substituted with two N-linked oligosaccharides, while only one is attached to each of the other alpha 1-microglobulins, and that the extra glycosylamine-linked oligosaccharide in the human protein is attached to asparagine in position 17. Finally it is shown that all four homologues inhibit antigen stimulation of human lymphocytes, a finding which is consistent with our previous suggestion that the N-linked oligosaccharides carry the immunosuppressive activity of alpha 1-microglobulin.