Partial Purification and Characterization of Native Plasminogen Activators from Bovine Milk

Abstract

At least four native plasminogen activators were detected in bovine milk, and two partially purified plasminogen activators were characterized. The plasminogen activators were dissociated from casein proteins by treatments with sulfuric acid and dimethylformamide. The plasminogen activators in the resulting fractions were partially purified with size exclusion, affinity, or metal chelate chromatographic techniques. Molecular weights of the two partially purified plasminogen activators were 47.2 and 30.5 kDa by gel electrophoresis. Size exclusion chromatography gave a molecular weight of 43.2 kDa for the first plasminogen activator. The isoelectric points of the two plasminogen activators were in the pH range 6.2 to 6.7. Because activity was not enhanced by the presence of fibrinogen fragments in a plasminogen activator assay mixture and decreased when human anti-urokinase Ig were added, at least some bovine milk native plasminogen activators appear to be urokinase-type plasminogen activators.

Effect of hydrolysis of casein by plasmin on the heat stability of milk

“…4:1 (Politis, Ng Kwai Hang, & Giroux, 1989). Plasminogen is activated to plasmin by a group of enzymes called plasminogen activators (PAs), which fall into two principal classes, urokinase-type (u-PA) and tissue-type (t-PA), and are associated with somatic cells and casein micelles, respectively (Deharveng & Nielsen, 1991;Lu & Nielsen, 1993a, b;Heegard, Rasmumssen, & Andreasen, 1994;White et al, 1995). Unlike plasmin, plasminogen and t-PA, which are associated with the casein micelles (Politis, Barbano, & Gorewit, 1992), the inhibitors of plasmin and of PAs occur in the serum phase of milk (Weber & Nielsen, 1991;Politis, 1996).…”

Effects of heat and β-lactoglobulin on distribution of fluorescently labeled tissue- and urokinase-type plasminogen activators in a model milk system

“…At pH 2-3, the ionic bound tPA can dissociate from casein micelles (Deharveng & Nielsen, 1991). Therefore, in comparison to ultracentrifugation, the distribution of tPA-647 in model milk systems fractionated by acid precipitation was examined.…”

UHT Milk Processing and Effect of Plasmin Activity on Shelf Life: A Review

“…Consequently, heat treatment of milk alters the natural balance between the activators and inhibitors in favor of the activators. This can lead to enhanced proteolysis in heated milk (Deharveng and Neilsen 1991). Driessen and van der Waals (1978) reported the D 142 °C for milk proteinase (plasmin) to be 18 s. Rollema and Poll (1986) reported 28%, 6%, 4%, and 1.3% plasmin/plasminogen remaining after indirect heating for 5 s at 110, 120, 140, and 147 °C, respectively, but none after heating at 147 °C for 10 s.…”

“…The role of PA in the system is to mediate plasminogen conversion into plasmin, whereas PAI and PI inhibit PA and plasmin activities, respectively. PAs are likely native to milk or produced by microorganisms (Deharveng and Nielsen 1991). Two major types of PA are known to be present in fresh milk: a tissue type associated with casein and a urokinase type associated with the somatic cells.…”