Disulfide structures of highly bridged peptides: A new strategy for analysis

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Abstract
A new approach is described for analyzing disulfide linkage patterns in peptides containing tightly clustered cystines. Such peptides are very difficult to analyze with traditional strategies, which require that the peptide chain be split between close or adjacent Cys residues. The water-soluble tris-(2-carboxyethyl)-phosphine (TCEP) reduced disulfides at pH 3, and partially reduced peptides were purified by high performance liquid chromatography with minimal thiol–disulfide exchange. Alkylation of free thiols, followed by sequencer analysis, provided explicit assignment of disulfides that had been reduced. Thiol–disulfide exchange occurred during alkylation of some peptides, but correct deductions were still possible. Alkylation competed best with exchange when peptide solution was added with rapid mixing to 2.2 M iodoacetamide. Variants were developed in which up to three alkylating agents were used to label different pairs of thiols, allowing a full assignment in one sequencer analysis. Model peptides used included insulin (three bridges, intra- and interchain disulfides; -Cys·Cys- pair), endothelin and apamin (two disulfides; -Cys·x·Cys- pair), conotoxin GI and isomers (two disulfides; -Cys·Cys- pair), and bacterial enterotoxin (three bridges within 13 residues; two -Cys·Cys- pairs). With insulin, all intermediates in the reduction pathway were identified; with conotoxin GI, analysis was carried out successfully for all three disulfide isomers. In addition to these known structures, the method has been applied successfully to the analysis of several previously unsolved structures of similar complexity. Rates of reduction of disulfide bonds varied widely, but most peptides did not show a strongly preferred route for reduction.