Distribution of plasminogen activator forms in different fractions of buffalo milk

In Italy buffalo milk is an important animal product utilized solely for the manufacture of Mozzarella cheese. Of an estimated population of 200000 buffalo there are ∼25000 controlled animals. The average milk production, expressed over 270 d lactation, is 2000 kg/head with average fat and protein contents of 82·6 and 46·4 g/l respectively (Associazione Italiana Allevatori, 1996). In recent years there has been a steady increase in the number of dairy buffalo replacing dairy cows as a consequence of the European Union quota system. The cheesemaking qualities of milk depend on many factors, the most important of which are the concentrations of intact casein and fat. Milk in which casein has been broken down by proteolytic enzymes is of less value to cheese manufacturers (Lucey & Kelly, 1994). Plasmin (EC 3.4.21.7), the most important endogenous milk proteinase, occurs in milk together with its inactive proenzyme, plasminogen (Schaar & Funke, 1986). Plasmin hydrolyses αs-casein and β-casein, although κ-casein has been reported to be resistant (Fox, 1981). However, Andrews & Alichanidis (1983) found κ-casein to be hydrolysed quite rapidly by plasmin. Plasmin activity is higher in mastitic than normal milk (Bastian & Brown, 1996). Stage of lactation affects plasmin activity: late lactation is associated with higher concentrations of plasmin (Gilmore et al. 1995; Baldi et al. 1996). Thus, plasmin could be a major problem in herds with seasonal breeding such as buffalo, which progress through lactation in synchrony and are therefore at a similar stage of lactation at a given time. The cascade of reactions leading to plasminogen activation is regulated by a complex network of molecular interactions between plasminogen activators (PA; EC 3.4.21.31) and at least three types of specific PA inhibitors (PAI-1; PAI-2; PAI-3; Saksela, 1985; Politis, 1996). There are two types of highly specific PA: tissue-PA (t-PA) and urokinase-PA (u-PA) (Saksela & Rifkin, 1988). PA activity is higher in mastitic than normal milk (Heegaard et al. 1994a). However, previous studies on PA in milk were carried out on bovine and caprine milk. No information is available on the presence and type of PA in buffalo milk. Thus the objective of the present study was to determine the level and type of PA in different fractions of buffalo milk: casein, serum, and somatic cells.