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.