Biosynthesis of rat alpha 1-macroglobulin. Identification of an intracellular precursor.

Alpha 1-macroglobulin was purified from rat plasma by gel filtration (Sephacryl S-300) and ion exchange chromatography (DE52). Analysis of the purified alpha 1-macroglobulin by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed two polypeptides: a light chain which could be resolved into a double band (36/38 kDa) and a heavy chain (160 kDa). Under non-reducing conditions complexes of 200 and 400 kDa could be demonstrated. Antibodies were raised against both chains of alpha 1-macroglobulin which did not cross-react with either rat alpha 2-macroglobulin or rat alpha 1-inhibitor 3. It was shown that in the medium of [35S]methionine-labeled hepatocytes the two subunits of alpha 1-macroglobulin are linked by disulfide bridges. Intracellularly, however, a high molecular mass polypeptide (185 kDa) could be immunoprecipitated with either the antiserum to the heavy or the light chain of alpha 1-macroglobulin, indicating the existence of a polyprotein precursor. Also in a cell-free translation system alpha 1-macroglobulin was synthesized as a polyprotein consisting of heavy and light chains (162 kDa). In a pulse-chase experiment using tunicamycin to block N-glycosylation, alpha 1-macroglobulin secretion was totally inhibited. This finding reflects the importance of the oligosaccharide side chains for the proteolytic processing to the two subunits and/or secretion of alpha 1-macroglobulin.