Subunit structure of the rat alpha-macroglobulin proteinase inhibitors

Rats produce 2 alpha-macroglobulin (alpha M) proteinase inhibitors, the alpha 1 M, normally found in the plasma, and the alpha 2 M, an acute phase protein. The alpha-macroglobulins were purified from the plasma of rats with adjuvant arthritis by polyethylene glycol precipitation, chromatography on a Zn2+ affinity column, and filtration on Sephacryl S-300 superfine. Comparison of the purified proteins on sodium dodecyl sulfate polyacrylamide gel electrophoresis following reduction reveals a 185 000 Da subunit for rat alpha 2 M identical to the human alpha 2 M, but a 167 000 plus a 38 000 Da subunit for rat alpha 1 M. Heat/alkali treatment (pH 11, 37 degrees C for 45 min) prior to reduction results in the appearance of 125 000 Da and 60 000 Da components from rat alpha 2 M analogous to the pattern of human alpha 2 M. In contrast, alpha 1 M showed in addition to the 125 000 Da band (and the unaltered 38 000 Da band), two bands of approx. 25 000 Da. Incubation with trypsin (approximately 1 mol/mol alpha M) prior to reduction causes formation of approximately 90 000 Da components from both rat inhibitors and the human alpha 2 M. The data suggests that only rat alpha 2 M and not rat alpha 1 M is structurally homologous to human alpha 2 M.