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

Abstract

SummaryWhole 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.