Inhibition of the angiotensin I converting enzyme (ACE) is an important therapeutic approach in the
treatment of several cardiovascular disorders. In this study the inhibitory potential of procyanidin-rich
fractions of grape seed (Vitis vinifera L.) was evaluated. The inhibitory capacity was assayed by fluorometric
determination of the amount of histidyl-leucine released from the substrate utilized in the
presence of ACE from rabbit lung. Four purified monomers [(þ)-catechin, ()-epicatechin, (þ)-catechin-
O-gallate, and ()-epigallocatechin] were also tested for comparison. Results showed that the natural
phenolic extracts exhibit higher inhibitory potential than the monomers, and that fractions with higher
average degrees of polymerization (DPn ¼ 8, IC50 ¼ 0.1 mg/L) present lower IC50 than the intermediate
ones (DPn ¼ 5, IC50 ¼ 3.5 mg/L). Such findings strengthen the idea that the inhibitor extension, and the
number of heterocyclic oxygen and hydroxyl groups in their structures, could define their in vitro action.