title: Heterogeneity of proteolytic enzyme activities in milk samples of different somatic cell count.
abstract: Milk contains the alkaline proteinase plasmin and lysosomal proteinases; the significance of the latter is ill-defined. The objective of this study was to investigate composition and activities of several different proteolytic enzymes in milk samples of varying somatic cell count (SCC). Increasing milk SCC was correlated with increased plasmin, cathepsin D and cysteine protease activities, with concomitant increases in proteolysis in milk. Addition of plasmin inhibitors confirmed the heterogeneity of proteinase activities in milk, as urea-PAGE analysis of milk samples showed casein hydrolysis in milk after 7 d storage even in samples with inhibitors added; extent and heterogeneity of proteolysis was correlated with milk SCC. Rennet coagulation properties were not significantly correlated with SCC, or activities of measured enzymes. Milk of increasing SCC also exhibited decreased physical stability during incubation of milk at 37 degrees C. Pasteurized milk was more stable than raw milk, suggesting that the enzyme(s) or mechanisms leading to such instability are impaired by pasteurization. Overall, milk has a very heterogeneous proteolytic enzyme population, with a higher significance of non-plasmin enzymes, such as cathepsin D and cysteine proteinases, than perhaps previously recognised.
authors.affiliation: Department of Food Science, Food Technology and Nutrition, University College Cork, Ireland.
authors.forename: Joanne M
authors.initials: JM
authors.lastname: Somers
authors.forename: Bernadette
authors.initials: B
authors.lastname: O'Brien
authors.forename: William J
authors.initials: WJ
authors.lastname: Meaney
authors.forename: Alan L
authors.initials: AL
authors.lastname: Kelly
mesh_terms: D000818:Animals; D002402:Cathepsin D / Q000378:metabolism; D002417:Cattle; D002452:Cell Count; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D010450:Endopeptidases / Q000378:metabolism*; D005260:Female; D005341:Fibrinolysin / Q000378:metabolism; D005519:Food Preservation; D006863:Hydrogen-Ion Concentration; D008892:Milk / Q000737:chemistry / Q000166:cytology* / Q000201:enzymology*; D008894:Milk Proteins / Q000032:analysis; D012044:Regression Analysis
chemical_list: D008894:Milk Proteins; D010450:Endopeptidases; D005341:Fibrinolysin; D003546:Cysteine Endopeptidases; D002402:Cathepsin D