Proteases and protein degradation in milk from cows infected with Streptococcus uberis

Proteolysis was studied in milk from cows experimentally inoculated with Streptococcus uberis into one quarter. Level of free amino terminals in quarter milk samples increased with somatic cell count (SCC) (P < 0.001). Tris-Tricine SDS-PAGE of the casein fraction showed a number of bands, including a fragment corresponding to alpha(sl)-I casein, found both in the casein fraction isolated from milk from infected glands and in casein from uninfected glands. Activity of cathepsin D was higher (P < 0.05) in milk from infected compared to uninfected quarters and increased with SCC (P < 0.01). Presence of cathepsins B and D in milk from infected quarters was shown by immunoblotting. Plasmin and bacterial proteases did not appear to be responsible for the apparent proteolysis occurring during storage. These results indicate that other proteases apart from the plasmin-system, e.g. lysosomal proteases, contribute significantly to the proteolysis in high cell count milk from S. uberis infected quarters. (C) 2004 Elsevier Ltd. All rights reserved.