Binding of .alpha.2-macroglobulin-thrombin complexes and methylamine-treated .alpha.2-macroglobulin to human blood monocytes

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Abstract
The binding of .alpha.2-macroglobulin (.alpha.2M) to human peripheral blood monocytes was investigated. Monocytes, the precursors of tissue macrophages, were isolated from fresh blood by centrifugal elutriation or density gradient centrifugation. Binding studies were performed using 125I-labeled .alpha.2M. Cells and bound ligand were separated from free ligand by rapid vacuum filtration. Nonlinear least-squares analysis of data obtained in direct binding studies at 0.degree.C showed that monocytes bound the .alpha.2M-thrombin complex with a Kd of 3.0 .+-. 0.9 nM and the monocyte had 1545 .+-. 153 sites/cell. Thrombin alone did not compete for the site. Binding was divalent cation dependent. Direct binding studies also demonstrated that monocytes bound methylamine-treated .alpha.2M in a manner similar to .alpha.2M-thrombin. Competitive binding studies showed that .alpha.2M-thrombinand methylamine-treated .alpha.2M bound to the same sites on the monocyte. In contrast, native .alpha.2M did not compete with .alpha.2M-thrombin for the site. Studies done at 37.degree.C suggested that after binding, the monocyte internalized and degraded .alpha.2M-thrombin and excreted the degradation products. Receptor turnover and degradation of .alpha.2M-thrombin complexes were blocked in monocytes treated with chloroquine, an inhibitor of lysosomal function. Our results indicate that human monocytes have a divalent cation dependent, high-affinity binding site for .alpha.2M-thrombin and methylamine-treated .alpha.2M which may function to clear .alpha.2M-proteinase complexes from the circulation.