We have characterized native and activated forms of rabbit alpha1M and compared them to rabbit and human alpha2M. Similar to human alpha2M, rabbit alpha1M is a tetramer associated via disulfide bonds and non-covalent interactions that exhibits autolysis into two fragments when heated. Like human alpha2M, rabbit alpha1M is cleaved by trypsin at one site; however, rabbit alpha1M shares characteristics with rabbit alpha2M that are different from the properties of human alpha2M. Amine or trypsin treatment of rabbit alpha-macroglobulins does not result in a significant conformational change or cleavage of four thiolester bonds. Full thiolester cleavage is only observed for rabbit alpha1M after exposure to both trypsin and a small amine. Additionally, amine-treated rabbit alpha-macroglobulins retain trypsin inhibitory potential and do not fully shield bound proteinases. Methylamine and trypsin treatment of rabbit alpha1M results in two dissimilar conformations that display differing exposure of the receptor-recognition site. While ammonia- and methylamine-modified rabbit alpha1M bind to macrophages with similar affinity to that of human alpha2M, trypsin-treated rabbit alpha1M exhibits dramatically lower affinity. This suggests that rabbit alpha1M may not play the same proteinase-inhibiting physiological role as human alpha2M.
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