DISTRIBUTION OF PLASMINOGEN-ACTIVATOR IN DIFFERENT FRACTIONS OF BOVINE-MILK

The type and relative amounts of plasminogen activator (PA) in differe nt fractions of bovine milk obtained from 15 Holstein cows were examin ed. Raw milk was centrifuged to separate skim milk and a somatic cell pellet. PA was mainly localized within the casein fraction, being 42 t imes that in the serum, and in association with somatic cells. The pre dominant form of PA in milk casein was isolated from SDS-PAGE gel extr acts and had a molecular mass of similar to 75 kDa. Its activity was i ncreased 4.1-fold (P < 0.01) in the presence of fibrin but was unaffec ted by the presence of amiloride, indicating that it was due to tissue -PA. The predominant forms of PA associated with milk somatic cells we re isolated from SDS-PAGE gel extracts and had molecular masses of sim ilar to 30 and similar to 50 kDa. The activity of both proteins was un affected by the presence of fibrin but was dramatically reduced by the presence of amiloride, indicating that they represented urokinase-PA.