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J. Dairy Sci. 2008. 91:2175-2183. doi:10.3168/jds.2007-0843
© 2008 American Dairy Science Association ®
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Plasmin Digestion of Photooxidized Milk Proteins

T. K. Dalsgaard*, C. W. Heegaard{dagger} and L. B. Larsen*,1

* Department of Food Science, University of Aarhus, Faculty of Agricultural Sciences, PO Box 50, DK-8830 Tjele, Denmark
{dagger} Protein Chemistry Laboratory, University of Aarhus, 8000 Aarhus C, Denmark

1 Corresponding author: LotteBach.Larsen{at}agrsci.dk

Plasmin-mediated hydrolysis of 6 different milk protein preparations [{alpha}S-casein ({alpha}S1 + {alpha}S2), β-casein, {kappa}-casein, {alpha}-lactalbumin, β-lactoglobulin, and lactoferrin] was found to be very dependent on photooxidation of the said proteins. Changes in plasmin proteolysis were investigated in a peptide-mapping study applying liquid chromatography-mass spectrometry. The changes were seen in the formation of peptides formed by plasmin-mediated hydrolysis after photooxidation, which was initiated with the naturally occurring photosensitizer riboflavin in all the milk protein preparations studied. The changes in the plasmin-mediated hydrolysis of photooxidized proteins are discussed in relation to changes introduced in the protein structure upon photooxidation. Plasmin-mediated hydrolysis of {alpha}S-casein, consisting of a mixture of {alpha}S1- and {alpha}S2-casein and a preparation of β-casein, was most highly affected by photooxidation, which is in agreement with the fact that those 2 proteins have been found to be most labile toward photooxidation. Changes in the formation of potential angiotensin-I-converting enzyme-inhibitory peptides as well as peptides proposed to have antibactericidal activities by plasmin were observed by oxidation of milk proteins before plasmin-mediated hydrolysis.

Key Words: photooxidation • plasmin proteolysis • protein oxidation • milk protein






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