A neutral subtilopeptidase inhibitor from porcine serum. Some evidence for .ALPHA.-marcroglobulin.

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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 homo-geneous 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 α-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 β-mercaptoethanol indicated that the inhibitor was made up of four subunit monomers having a molecular weight of 190, 000. From comparisons of its physicochemical and inhibitory properties with those of well-investigated plasma proteins, the inhibitor was identified as α2-macroglobulin. On treatment of the inhibitor with neutral subtilopeptidase, a protein with a molecular weight of 95, 000 appeared after treatment with SDS and β-mercaptoethanol, suggesting that a peptide bond susceptible to the enzyme exists near the mid-point of the subunit chains.

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