The alpha-macroglobulins from rat plasma: structure, plasma clearance and endocytosis of complexes with subtilisin

Rat alpha 2M was isolated from plasma of animals which had been treated with stress hormones. Polyacrylamide-gel electrophoresis of the reduced protein in the presence of sodium dodecyl sulfate showed a band with a Mr of 175,000. In a previous work (Biochem. J. 226, 75-84, 1985) we had found that rat alpha 1M consists of two types of subunits with Mr's of 163,000 and 37,000. Like the large subunit of alpha 1M the chain of alpha 2M contained a peptide bond that was susceptible to autolysis and a site that was split by trypsin. Negatively stained free alpha 2M could be distinguished from free alpha 1M under the electron microscope. Radioactively labelled alpha 2M was slowly cleared from plasma, but complexes of the protein with labelled subtilisin were rapidly cleared with first-order kinetics, showing a half-life of about 6 min. These complexes were mainly taken up by the liver, as had previously been described for alpha 1M-subtilisin complexes. Complexes of both macroglobulins competed with each other for uptake, indicating endocytosis via a common receptor.