The effect of polyamines on the digestion of proteins by serine proteases was examined. Based on the mechanism of action of serine proteases, it was anticipated that nucleophilic functionalities such as amino groups in polyamine, rather than hydroxyl ions, would react with peptide bonds during the hydrolysis process. If this were the case, polyamine might be covalently linked to the C-terminus of the product peptides during protein digestion. In order to test this hypothesis, hemoglobin was used as a model protein and was digested with trypsin in the presence of polyamine. The product peptides were separated, collected by HPLC, and analyzed by MALDI-TOF MS using post-source decay. The results showed that some peptides were indeed modified with polyamine at the C-terminus. (c) 2007 Elsevier Inc. All rights reserved.