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Identification of peptides in milk as a result of proteolysis at different levels of somatic cell counts using LC MALDI MS/MS detection

Published online by Cambridge University Press:  29 January 2008

Anna Wedholm*
Affiliation:
Department of Food Science, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden
Hanne S Møller
Affiliation:
Department of Food Science, Danish Institute of Agricultural Sciences, Research Centre Foulum, DK-8830 Tjele, Denmark
Helena Lindmark-Månsson
Affiliation:
Swedish Dairy Association, SE-223 70 Lund, Sweden
Morten D Rasmussen
Affiliation:
Department of Animal Health, Welfare and Nutrition, Danish Institute of Agricultural Sciences, Research Centre Foulum, DK-8830 Tjele, Denmark
Anders Andrén
Affiliation:
Department of Food Science, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden
Lotte B Larsen
Affiliation:
Department of Food Science, Danish Institute of Agricultural Sciences, Research Centre Foulum, DK-8830 Tjele, Denmark
*
*For correspondence; e-mail: Anna.Wedholm@lmv.slu.se

Abstract

The somatic cell count (SCC) in milk is associated with increasing proteolytic degradation of caseins and it has been suggested that enzymes derived from somatic cells contribute to a lower yield and poorer quality of cheese. It is essential to increase the knowledge on naturally occurring milk proteinase activities to better understand how to improve the technological quality of milk. The aim of this work was to identify peptides actually present in milk as a result of proteolysis at different levels of SCC and to assign these peptides to potential responsible proteases where possible. Peptide fractions were prepared from acid whey by ultrafiltration at a molecular cut-off value of 10 000 Da. The peptides were separated using capillary reversed phase high performance liquid chromatography (RP-HPLC) and identified by matrix-assisted laser desorption/ionization-time of flight tandem mass spectrometry (MALDI-TOF MS/MS). Peptides identified ranged in mass from 1023 to 2000 Da, and originated from αS1-, αS2- or β-casein. Possible responsible proteases that could be suggested when examining the peptide cleavage sites included plasmin, cathepsin B, D and leukocyte elastase. The results indicated that plasmin was primarily responsible for the observed proteolysis in milk at low cell count, whereas the cathepsins and elastase became implicated at elevated cell count. Specificity and activity of cathepsins and elastase has earlier mainly been studied in model systems, whereas less is known about their activities in milk itself. This is also the first indication of involvement of elastase in milk proteolysis through the unequivocal determination of cleavage sites.

Type
Research Article
Copyright
Copyright © Proprietors of Journal of Dairy Research 2008

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