Detection of traces of a trisulphide derivative in the preparation of a recombinant truncated interleukin-6 mutein.

A new mutein of interleukin-6, called delta 22-IL-6 Cys 3,4, characterized by the deletion of the first 22 amino acids at the N-terminal end and by the substitution of the first two cysteines (Cys23 and Cys29) with serine residues, was produced in Escherichia coli and was found to maintain the structural and functional properties of the human native form. A partially purified preparation still showed in isoelectric focusing a minor acidic component (pI 6.10) and a more basic component (pI 6.70), the native form having a pI of 6.56. This preparation was further fractionated in a multi-compartment electrolyser with isoelectric membranes, which allowed the collection of the more alkaline species for characterization. Mass spectra of the pI 6.70 form gave an additional mass of 32 atomic mass units (amu), suggesting the addition of two oxygen atoms (a potential oxidation of two methionine residues to sulphoxide). However, the five methionine residues in this higher pI form were identified after enzymatic hydrolysis and peptide mapping and were found to be in a reduced state. In addition, the pI 6.70 form was quickly converted into the native form by mild reductive treatment. On digestion and fingerprinting, the peptide from residues 50 to 65 of the pI 6.70 species (containing the only two cysteine residues of the molecule) exhibited a more hydrophobic behaviour in reversed-phase high-performance liquid chromatography and retained a mass increase of 32 amu. These experimental findings more likely suggest the addition of an extra sulphur atom to the only disulphide bridge to give an unusual protein trisulphide molecule.