Bovine erythrocyte superoxide dismutase. Subunit structure and sequence location of the intrasubunit disulfide bond.

The subunit assembly of bovine erythrocyte superoxide dismutase has been studied, through analysis of the effects of heat, removal of metal ion cofactors, and disulfide bond reduction, upon the dissociation of the subunits. Investigations utilizing sodium dodecyl sulfate-acrylamide gel electrophoresis and sedimentation equilibrium analysis demonstrate that the 2 polypeptide subunits of the protein are not covalently joined, but are associated through unusually strong noncovalent interactions. This absence of intersubunit disulfide bonds, in conjunction with the titration of a single free sulfhydryl group per subunit polypeptide chain, necessitates that the 3 half-cystine residues per subunit exist as 1 residue of cystine, forming an intrasubunit disulfide bond, and 1 residue of cysteine. The specific half-cystine residues participating in the disulfide linkage and contributing the free thiol function were identified. Analysis of the products of cyanogen bromide cleavage, with acrylamide gel electrophoresis, demonstrates that the intrasubunit disuhide links 1 half-cystine residue in the sequence NHz-terminal to Met-115, to another halfcystine residue in the sequence COOH-terminal to Met-115. Through conventional procedures of enzymatic digestion, peptide isolation, and peptide characterization, Cys-55 and Cys-144 are shown to participate in the intrasubunit disulfide linkage, and Cys-6 to exist as free cysteine.