Mutagenic contaminants in synthetic peptides obtained by an azide coupling

Hormone-like peptides are, almost by definition, not mutagenic. It was, therefore, unusual to find that some batches of peptides synthesized by azide coupling were mutagenic in the Ames test. One of these peptides, eledoisin, showed mutagenic activity particularly in Salmonella typhimurium TA 1535 without metabolic activation. This activity was independent of the peptide purity determined by HPLC and a dose response relationship was observed at concentrations over the solubility limit of the peptide in the assay medium. We therefore suggested that the mutagenic effect might be due to the presence of chemically undetectable, water-soluble impurities, which could be removed by counter-current distribution. If, however, the same final coupling was carried out by the mixed anhydride procedure, no mutagenic activity was observed. Consequently, we considered that the mutagenicity detected was due to traces of hydrazoic acid salts arising during azide formation in the coupling step. In fact only the product of the coupling reaction between the pivotal intermediates was mutagenic.