Fractionation and partial characterization of alpha-1-protease isoinhibitors of horse.

The principal alpha-1-protease inhibitor of horse was fractionated by classical methods and analysed with a modified fibrinogen-agarose gel electrophoretic method of high sensitivity and resolving power. Starting with an electrophoretically homogeneous inhibitor in unfractionated serum, two isoinhibitor bands became apparent after fractionation with (NH4)2SO4 and DEAE-cellulose DE-52 ion-exchange chromatography. The isoinhibitors differed in electrophoretic migration and in the elution pattern from Sephadex G-100 gel filtration, but possessed identical antigenic determinants and enzyme specificity. The slower migrating isoinhibitor with an apparent molecular weight of 90 000 could be highly purified. In contrast the faster moving isoinhibitor (molecular weight 65 000) could not be completely freed from a contaminating alpha-2-protease inhibitor. The formation of the two isoinhibitors is discussed considering conformational changes analogous to phenomena observed with alpha-2-macroglobulin, or dimer formation in combination with altered conformations. The isoinhibitors described here are new additions to the different heterogeneities which exist in alpha-1-protease inhibitors in horse. They also supplement the different heterogeneities which exist among the alpha-1-protease inhibitors of mammals.