ORIGINAL ARTICLE The impact of photo-induced molecular changes of dairy proteins on their ACE-inhibitory peptides and activity Barbara Kerkaert • Fre ´de ´ric Mestdagh • Tatiana Cucu • Kshitij Shrestha • John Van Camp • Bruno De Meulenaer Received: 14 July 2011 / Accepted: 9 November 2011 Ó Springer-Verlag 2011 Abstract Among all dietary proteins, dairy proteins are the most important source of bio-active peptides which can, however, be affected by modifications upon processing and storage. Since it is still unknown to which extent the bio- logical activity of dairy proteins is altered by chemical reactions, this study focuses on the effect of photo-induced molecular changes on the angiotensin I converting enzyme (ACE) inhibitory activity. Milk proteins were dissolved in phosphate buffer containing riboflavin and stored under light at 4°C for one month during which the molecular changes and the ACE-inhibitory activity were analysed. An increase in the total protein carbonyls and the N-formyl- kynurenine content was observed, besides a decrease in the free thiol, tryptophan, tyrosine and histidine content. These changes were more severe in caseins compared with whey proteins and resulted moreover in the aggregation of caseins. Due to these photo-induced molecular changes, a significant loss of the ACE-inhibitory activity was observed for casein peptides. A peptide analysis moreover illustrated that the decreased activity was not attributed to a reduced digestibility but to losses of specific ACE-inhibi- tory peptides. The observed molecular changes, more specifically the degradation of specific amino acids and the casein aggregation, could be assigned as the cause of the altered peptide pattern and as such of the loss in ACE- inhibitory activity. Keywords Milk proteins  Photo-oxidation  ACE-inhibitory activity  ACE-inhibitory peptides  LC–TOF–MS Abbreviations ACE Angiotensin I converting enzyme DNPH 2,4-Dinitrophenylhydrazine DTNB 5,5-Dithiobis(2-nitro-benzoic acid) OPA Ortho-phthaldialdehyde FMOC 9-Fluorenylmethylchloroformate NFK N-formylkynurenine ALA a-Lactalbumin BLG b-Lactoglobulin BSA Bovine serum albumin Introduction The importance of dairy proteins in the human diet has been recognized for many decades in view of their nutri- tional quality, versatile functionalities and more recently also for their physiological activities (Korhonen et al. 1998). The two major classes of milk proteins, whey (20%) and caseins (80%) differ greatly in their physicochemical properties, but are, however, both complex proteins, con- taining high levels of essential amino acids and having a high protein efficiency ratio (Hoffman and Falvo 2004). Besides, both types of milk proteins are known as major sources of bioactive peptides with a wide range of physi- ological activities (Hartmann and Meisel 2007; Korhonen Electronic supplementary material The online version of this article (doi:10.1007/s00726-011-1157-y) contains supplementary material, which is available to authorized users. B. Kerkaert  F. Mestdagh  T. Cucu  K. Shrestha  J. Van Camp  B. De Meulenaer (&) NutriFOODchem unit (partner in Food2Know), Department of Food Safety and Food Quality, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, 9000 Ghent, Belgium e-mail: Bruno.DeMeulenaer@UGent.be 123 Amino Acids DOI 10.1007/s00726-011-1157-y