Interaction of β-Lactoglobulin with Small Hydrophobic Ligands - Influence of Covalent AITC Modification on β-LG Tryptic Cleavage

Covalent modification of proteins with bioactive organosulfur compounds is suggested to reduce the smell and increase the stability of the compound. Aside from these effects the covalent modification may also alter the physiochemical properties of the protein. In this study, the whey protein β-lactoglobulin (β-LG) was covalently modified with bioactive organosulfur compound allyl isothiocyanate (AITC), originating from cabbage. Native and AITC modified β-LG were subjected to tryptic and chymotryptic digestion to assess the influence of the covalent modification on peptide formation. AITC was shown to modify at least 13 different amino acid residues, containing thiol- or amino groups, in a concentration dependent manner. Therefore, AITC modification can be controlled to some extent. Besides cysteine thiols, the most accessible amino groups for AITC modification were found at the N-terminal end of the protein (residues L1 and K8) along with lysine residues K91, K77 and K83. Higher amount of AITC addition resulted in a significant blocking of several tryptic cleavage sites of β-LG (for example residue K14) resulting in longer peptides, influencing the concentration of certain bioactive peptides following tryptic cleavage.