Enzymically Catalyzed Disulfide Interchange in Randomly Cross-linked Soybean Trypsin Inhibitor

Abstract

SUMMARY The protein disulfide interchange enzyme, isolated from beef liver microsomes, has been further purified by gradient elution from diethylaminoethyl-Sephadex columns and by starch gel electrophoresis. The purified enzyme catalyzed the reactivation of fully reduced soybean trypsin inhibitor in the presence of either a soluble, dialyzable, fraction of liver homogenate, or dehydroascorbic acid. However, as described previously, the enzyme appears to be concerned only with the second stage of this process, namely the rearrangement of incorrectly paired half-cystine residues introduced during the oxidative step. Randomly cross-linked soybean trypsin inhibitor, prepared by oxidation of fully reduced material in 8 M urea, is rapidly con- verted to the native protein in the presence of mM P-mercapto- ethanol at a molar ratio for enzyme to protein substrate of 1:360. The present experiments establish the enzymic nature and generality of the reaction. Thus, randomly cross-linked bovine pancreatic ribonuclease and egg white lysozyme, both animal proteins, and soybean trypsin inhibitor, a plant protein, can all serve as substrates for the enzyme. Acknowledgment-We wish to thank Dr. K. Hoerman for his kind assistance with the experiments involving the purification of the enzyme by starch gel electrophoresis. REFERENCES 1.