Oligo-tyrosine peptides with degrees of polymerization ranging from 2 to 5 could be synthesized by alpha-chymotrypsin-catalyzed reaction with L-tyrosine ethyl ester in aqueous media, although the peptide yield was low due to a preferential hydrolysis of the substrate. It was also confirmed that alpha-chymotrypsin efficiently converted tyrosine tetramer to the dimer which was resistant to the digestion. Both Tyr-Tyr and Tyr-Tyr-Tyr showed high inhibitory activity for angiotensin I-converting enzyme from rabbit lung, and their IC50 values were 34 mu M and 51 mu M, respectively. These two peptides exhibited a mix of competitive and noncompetitive inhibitions. Tyr-Tyr-Tyr was first recognized as an ACE inhibitor, suggesting that alpha-chymotrypsin Could be applied to synthesis of novel potential materials for antihypertensive medicines. (C) 2008 Elsevier Inc. All rights reserved.