In this study, the cleavage of protein by molybdenum cluster is reported for the first time. The protein target used is porcine pepsin. The data presented in this study show that pepsin is cleaved to at least three fragments with molecular weights of similar to 23, similar to 19 and similar to 16 kDa when the mixture of the protein and ammonium heptamolybdate tetrahydrate ((NH4)(6)Mo7O24.4H(2)O) was incubated at 37 degrees C for 24 h. No self cleavage of pepsin occurs at 37 degrees C, 24 h indicating that the reaction is mediated by the metal ions. N-terminal sequencing of the peptide fragments indicated three cleavage sites of pepsin between Leu 112-Tyr 113, Leu 166-Leu 167 and Leu 178-Asn 179. The cleavage reaction occurs after incubation of the mixture of pepsin and (NH4)(6)Mo7O24.4H(2)O) only for 2 h. However, the specificity of the cleavage decreases when incubation time is longer than 48 h. The mechanism for cleavage of pepsin is expected to be hydrolytic chemistry of the amide bonds in the protein backbone. (C) 2012 Elsevier Inc. All rights reserved.