The impact of photo-induced molecular changes of dairy proteins on their ACE-inhibitory peptides and activity

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 biological 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-formylkynurenine 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-inhibitory 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.