Isolation and characterization of goat serum α1-globulin protease inhibitors

Abstract

α1-Globulin-type protease inhibitors were isolated from goat serum by two methods, namely preparative isoelectric focusing and preparative electrophoresis in polyacrylamide gel. The fractions obtained by the first method showed varying isoprotein compositions by analytical isoelectric focusing. Sodium dodecyl sulfate – polyacrylamide gel electrophoresis (SDS–PAGE) revealed the presence of one protein in the fractions with the same velocity of migration as purified human α1-antitrypsin and a second protein with a slightly higher migration velocity. The ratios of trypsin-inhibiting to chymotrypsin-inhibiting capacities in all the fractions were the same and both inhibitors were stable upon storage. The reaction of the inhibitors with trypsin and chymotrypsin was also demonstrated by analytical isoelectric focusing.The fractions obtained by preparative gel electrophoresis (the second method) contained the same proteins but their proportions varied widely in different fractions as demonstrated by analytical electrofocusing in the presence of urea and by SDS–PAGE. The early fractions, which consisted predominantly of α1-antitrypsin, showed a high inhibiting capacity for trypsin and none or only negligible capacity for chymotrypsin. Conversely, in the late fractions, the proportions of the proteins and inhibiting capacities were reversed. At 4 °C the trypsin-inhibiting capacity was stable for weeks but the chymotrypsin-inhibiting capacity of the preparation rapidly decreased.These observations indicate that the inhibition of proteases by goat α1-globulins is due to at least two closely associated but distinguishable proteins. One of these, corresponding to human α1-antitrypsin, would have an appreciable capacity to inhibit trypsin, but unlike the latter, little or no capacity for chymotrypsin inhibition. The inhibition of chymotrypsin is due to the second, unidentified α1-globulin.