Variation in activity and heterogeneity of bovine milk proteases with stage of lactation and somatic cell count

Samples of prepartum secretion, colostrum and mature milk were analysed for variation in activity of cathepsin D and cysteine protease due to lactation stage and somatic cell count (SCC). In spite of considerable variation between animals, cathepsin D activity was found to be highest in prepartum secretion, lowest in colostrum and at intermediate levels in milk throughout lactation. Levels of cysteine protease activities were highest 1 week postpartum and lowest in early/mid-lactation milk. The activity of an unidentified milk protease with specificity different from cathepsin D and cysteine proteases was also detected in milk, but had very low activity in prepartum secretions and colostrum. By immunoblotting, prepartum secretion was found to contain both cathepsin D and procathepsin D, while only procathepsin D could be detected in colostrum and milk. Cathepsin D activity was positively correlated with SCC, and immunoblotting indicated that the elevated activity derived from cathepsin D was due to increased level of immunoreactive procathepsin D in high SCC milk, and not to increased levels of mature cathepsin D. This indicates that activation of procathepsin D does not occur at elevated SCC.