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Rat blood plasma contains three high molecular weight thiol ester-containing proteinase inhibitors, alpha 1-macroglobulin (alpha 1M), alpha 1-inhibitor III (alpha 1I3), and alpha 2-macroglobulin (alpha 2M). Rat serums have been analyzed using a two-dimensional gel electrophoretic technique which optimizes recovery of high molecular weight proteins. alpha 1M, and (alpha beta)4-tetramer in native solution, separated in the second sodium dodecyl sulfate-containing electrophoretic dimension as a disulfide-linked (alpha beta)2-dimer with an approximate Mr of 360 kDa. alpha 1I3 separated in the gels as a single 190-kDa polypeptide. It is also a monomer in native solution by ultracentrifugation criteria. Native rat alpha 2M is a tetramer, but it separates in the gels as a disulfide-linked dimer with an Mr of approximately 360 kDa. The kinetics of changes in concentration of these proteins during the induction of polyarthritis was also measured by quantitative immunoelectrophoresis. In rats with adjuvant-induced polyarthritis, the concentration of alpha 1I3 dramatically decreases and alpha 2M appears and continues to increase in a biphasic manner for 2 weeks. The alpha 1M concentration remains relatively constant. All three macroglobulins were purified utilizing modern rapid chromatographic techniques, and parallel comparisons of their native physicochemical properties were carried out. The N-terminal sequence of the alpha-chain of rat alpha 1M was also shown to share sequence homology with that of alpha 2M. In agreement, Esnard et al. (Esnard, F., Gutman, N., El Moujahed, A., and Gauthier, F. (1985) FEBS Lett. 182, 125-129) recently reported that alpha 1I3 also contains a thiol ester bond, as do alpha 1M and alpha 2M, since it reacts covalently with [14C]methylamine and is cleaved autolytically at 80 degrees C. We have examined negatively stained preparations of native, trypsin-treated, and methylamine-treated human alpha 2M, rat alpha 2M, and rat alpha 1M in the electron microscope. Trypsin appears to convert globular ring-shaped native molecules to rectangular box-like structures, in agreement with the conclusions of a recent report on human alpha 2M (Tapon-Bretaudiere, J., Bros, A., Couture-Tosi, E., and Delain, E. (1985) EMBO J. 4, 85-89).
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