M. Aslam et Wl. Hurley, PROTEOLYSIS OF MILK-PROTEINS DURING INVOLUTION OF THE BOVINE MAMMARY-GLAND, Journal of dairy science, 80(9), 1997, pp. 2004-2010
The role of proteolytic hydrolysis of milk proteins in the mammary gla
nd during involution is unknown. The objectives of this study were to
determine the activities of plasmin, plasminogen, and plasminogen acti
vator in mammary gland secretions collected during involution and to i
dentify peptides generated by proteolysis of casein and lactoferrin in
those secretions. Mammary secretions were collected from Holstein cow
s on d 7, 14, and 21 of involution and on d 7 postcalving. Activities
of plasmin, plasminogen, and plasminogen activator were determined on
the defatted, filtered aqueous phase of mammary secretions. Activities
of plasmin, plasminogen, and plasminogen activator were significantly
higher on d 7, 14, and 21 of involution than were those on d 7 postca
lving. Protein fragments resulting from hydrolysis were detected by SD
S-PAGE in samples collected on d 7, 14, and 21 of involution, but few
protein fragments were observed in samples collected on d 7 postcalvin
g when plasmin activity was low. Immunoblot analysis showed that a num
ber of peptides observed during involution were generated from alpha(s
)-casein (CN), beta-CN, kappa-CN, or lactoferrin, The appearance of pe
ptides from proteins of mammary secretions during early involution was
generally correlated with increased plasmin activity. Elevated plasmi
n activity during mammary involution may be primarily responsible for
the observed concurrent hydrolysis of milk proteins in mammary secreti
ons.