Purification of the human placental alpha 2-macroglobulin receptor

The alpha 2-macroglobulin receptor was solubilized from human placental membranes, purified and characterized. Affinity cross-linking of labelled ligand to intact membranes showed a receptor size compatible with 400-500 kDa. The membranes were solubilized in 3-[(3-cholamidopropyl)dimethylammonio]propane sulfonate (CHAPS) and affinity chromatography was performed using Sepharose-immobilized alpha 2-macroglobulin-methylamine with elution in buffer containing 2 mM EDTA, pH 6.0. SDS-PAGE of the resulting receptor preparation showed a predominant approx. 440 kDa band (reducing conditions) and some minor accompanying proteins of 70-90 kDa and 40 kDa. The yield was 400-800 micrograms receptor preparation per placenta. The receptor preparation immobilized on nitrocellulose bound the alpha 2-macroglobulin-trypsin complex with a dissociation constant of about 400 pM. 125I-iodinated receptor preparation bound almost quantitatively to Sepharose-immobilized alpha 2-macroglobulin-methylamine in the presence of CHAPS alone, and bound 70-80% in the presence of 0.2% SDS. The labelled proteins were separated in the presence of 0.2% SDS by gel filtration or SDS-PAGE (unboiled samples). The 440 kDa protein accounted for the major part of the binding, although some approx. 80 kDa proteins, perhaps proteolytic degradation products, also showed binding activity.