Incubation of sodium caseinate (NaCN) and purified alpha-casein (alpha(s)-CN) with an Aspergillus niger derived prolyl endoproteinase (An-PEP) for 1, 2, 3, 4, 8 and 24 h resulted in the generation of potent angiotensin converting enzyme (ACE) inhibitory hydrolysates. An ACE IC50 of 21.1 +/- 5.1 mu g/ml was obtained on incubation of An-PEP with NaCN for 4 h. Fractionation of the NaCN hydrolysates using 3 kDa centrifugal filters resulted in highly active permeate fractions, the most potent being obtained from the 3 h hydrolysate (ACE IC50 = 2.9 +/- 0.3 mu g/ml). The hydrolytic specificity of An-PEP for purified alpha-CN was assessed using UPLC ESI MS/MS. The analysis confirmed An-PEP's cleavage preference for the C-terminal side of Pro and also confirmed that An-PEP has the ability to cleave at the C-terminal of Ala, Leu, Arg and His residues. (C) 2015 Elsevier Ltd. All rights reserved.