UV-C light promotes the reductive cleavage of disulfide bonds in β-Lactoglobulin and improves in vitro gastric digestion

Abstract

β-Lactoglobulin (β-Lg) is the main protein in whey and is known for its allergenicity and resistance to the digestion of pepsin and trypsin. The UV-C photoinduced cleavage of disulfide bonds in β-Lactoglobulin, as promoted by excitation of tryptophan residues (Trp), is shown to induce changes in the protein's secondary structure, significantly reducing the protein's resistance to pepsin digestion. The UV-C light-induced changes in the protein secondary structure are marked by an increase in the contribution of β-sheet and α-helix structures with a concomitantly smaller contribution of the β-turn structural motif. The photoinduced cleavage of disulfide bonds in β-Lg has an apparent quantum yield of ф = 0.0015 ± 0.0003 and was shown by transient absorption laser flash photolysis to arise by two different pathways: a) the reduction of the disulfide bond Cys66Cys160 occurs by direct electron transfer from the triplet-excited 3Trp to the disulfide bond due to the existence of a CysCys/Trp triad (Cys66Cys160/Trp61) and b) the reduction of the buried Cys106Cys119 disulfide bond involves a reaction with a solvated electron originated by the photoejection of electrons from the triplet-excited 3Trp decay. The in vitro gastric digestion index for UV-C-treated β-Lg is revealed to have increased significantly by 36 ± 4 % and 9 ± 2 % under simulated elderly and young adult digestive conditions, respectively. When compared to the native protein, the peptide mass fingerprint profile of digested UV-C-treated β-Lg shows a higher content and variety of peptides, including the production of some exclusive bioactive peptides such as PMHIRL and EKFDKALKALPMH.