Purification and Properties of an Enzyme from Beef Liver Which Catalyzes Sulfhydryl-Disulfide Interchange in Proteins

Abstract An enzyme which catalyzes sulfhydryl-disulfide interchange in proteins containing incorrect disulfide bonds has been isolated from beef liver microsomes. The purified form of this enzyme shows two bands upon electrophoresis in starch gels and polyacrylamide gels. The proteins represented by these bands are considered to be variants of the same protein because both are enzymically active after elution from starch gels and because they appear as a single band after reduction and alkylation. A contaminant of higher molecular weight was identified in the purified enzyme preparation both as a faint third band in starch and polyacrylamide gels and as a faster sedimenting component in the analytical ultracentrifuge. By the latter technique the quantity of the contaminant was estimated to be 8 to 10% that of the microsomal enzyme. The molecular weight of the enzyme was found to be 42,000 by analytical ultracentrifugation. Amino acid analyses of the enzyme disclosed the presence of 3 half-cystine residues, and 44 arginine plus lysine residues, per molecule. Peptide maps, prepared from tryptic digests of the enzyme, contained 48 ninhydrin-positive spots. The visible and ultraviolet absorption spectra of the enzyme showed no peaks other than one with a maximum at 278 mµ.