Subunit Structure of the Rat α-Macroglobulin Proteinase Inhibitors

Abstract

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 .**GRAPHIC**. 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 [dalton] 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.2 M. Heat/alkali treatment (pH 11, 37.degree. 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 38000 Da band), 2 bands of .apprx. 25,000 Da. Incubation with trypsin (.apprx. 1 mol/mol .alpha.M) prior to reduction causes formation of .apprx. 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.