Casein compositional studies: II. The effect of secretory disturbance on casein composition in freshly drawn and aged bovine milks

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Abstract
Summary: Whole casein preparations obtained from paired quarters (one healthy and one subclinically infected) of each of 6 individual cows were fractionated quantitatively by column chromatography on hydroxyapatite. Infection was associated with an apparent decrease in the αs- and β-casein fractions and increase in the γ- and κ-casein fractions. The increase in the κ-casein fraction was attributable mainly to enrichment with chymosin-resistant minor components of casein.The profile produced by column chromatography on DEAE-cellulose of casein isolated from a healthy quarter of a single cow differed significantly from that from a subclinically infected quarter of the same cow. These differences were increased by incubation of the quarter milks at 37 °C for 8 h. Extensive fragmentation of [14C]methylated αs2- and β-casein, and less extensive fragmentation of [14C]methylated αs1-casein, occurred during incubation of these proteins in milk from an infected quarter. Fragmentation was almost entirely inhibited by soybean trypsin inhibitor. From a consideration of the chromatographic behaviour of the radiolabeled fragments on DEAE-cellulose, and other data presented, it was concluded that proteolysis of caseins by a plasmin-like enzyme was sufficient to account, in qualitative terms, for most of the compositional changes exhibited by casein from subclinically infected quarters.