Distribution of plasminogen and plasmin in fractions of bovine milk.

The relative amounts of immunoreactive plasminogen and active plasmin in different fractions of bovine milk were examined. Raw milk was centrifuged to separate skim, cream, and a somatic cell pellet. Skim milk was centrifuged to separate milk serum and casein micelles. Milk fat globule membranes were isolated from the cream fraction of bovine milk. Proteins from somatic cells were isolated following sonication of the cells. Western blot analysis showed the presence of several forms of plasminogen in bovine milk. The predominant forms of plasminogen identified following electrophoresis under nonreducing conditions were proteins with approximate molecular weights of 88,000, 152,000, and 160,000. The predominant forms of plasminogen identified after electrophoresis under reducing conditions were two proteins with approximate molecular weights of 88,000 and 50,000. The highest amount (82% of the total plasminogen), as determined by an ELISA, was associated with the casein fraction. Lower plasminogen concentrations were associated with the serum, cream fractions, and milk fat globule membranes. The SDS-PAGE of the cream and milk fat globule membranes indicated that some casein was present in both fractions. Thus, the low plasminogen concentrations in these fractions may be associated with the caseins there. No immunoreactive plasminogen was present in the somatic cells. Active plasmin was present in the same milk fractions in which plasminogen was detected: casein, serum, and cream.