Rat alpha 1-macroglobulin (alpha 1M), rat alpha 2-macroglobulin (alpha 2M) migrated as single bands on non-denaturing gels when purified by the methods described. All three proteins demonstrated increased mobility after reaction with trypsin. A single saturable pathway rapidly cleared complexes of trypsin and the alpha-macroglobulins of mouse, rat and human from the circulation of mice. None of the native alpha-macroglobulins competed for clearance with the trypsin complexes. [14C]Methylamine incorporation was 4.1, 3.9, 2.6 and 3.2 mol/mol of proteinase inhibitor for human alpha 2M, rat alpha 1M, rat alpha 2M and mouse alpha 2M, respectively. Only rat alpha 2M, the acute-phase alpha-macroglobulin studied, showed no evidence of conformational change when subjected to electrophoresis after reaction with methylamine. The clearance of rat alpha 2M-methylamine was comparable with that of the native molecule. The other alpha-macroglobulin-methylamine complexes cleared faster than the inhibitors that had not reacted. Rat alpha 2M and rat alpha 2M-methylamine bound equivalent quantities of 1251-labelled trypsin (1.01 and 0.96 mol/mol respectively). The soya-bean trypsin inhibitor-resistant esterolytic activity of trypsin bound to rat alpha 2M-methylamine was approx. 90% suppressed compared with proteinase bound to native rat alpha 2M. This suppression was not due to a change in the affinity of soya-bean trypsin inhibitor for the complex. Reaction of rat alpha 2M-methylamine with trypsin resulted in a ‘slow’ to ‘fast’ electrophoretic conversion of the proteinase inhibitor, and exposure of the signal on the alpha 2M that causes the complex to clear from the murine circulation.

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