A neutral subtilopeptidase inhibitor from porcine serum some evidence for alpha2-macroglobulin

An inhibitor of neutral subtilopeptidase [EC 3.4.24.4] was purified from porcine serum by salting out with (NH4)2SO4, chromatography on anion exchange sephadex, gel filtration with Sepharose 6B, and isoelectric focusing. The preparation was homogeneous by electrophoretic and ultracentrifugal criteria, and was shown to be a glycoprotein with a molecular weight of 740,000. It inhibited the caseinolytic activities of thermolysin, subtilisin, trypsin [EC 3.4.21.4], and alpha-chymotrypsin [EC 3.4.21.1] as well as that of neutral subtilopeptidase by an equimolar binding to those proteolytic enzymes. SDS-polyacrylamide gel electrophoresis after reduction with beta-mercaptoethanol indicated that the inhibitor was made up of four subunit monomers having a molecular weight of 190,000. From comparisons of its physiocochemical and inhibitory properties with those of well-investigated plasma proteins, the inhibitor was identified as alpha2-macroglobulin. On treatment of the inhibitor with neutral subtilopeptidase, a protein with a molecular weight of 95,000 appeared after treatment with SDS and beta-mercaptoethanol, suggesting that a peptide bond susceptible to the enzyme exists near the mid-point of the subunit chains.