As the demands for food products of high quality increase, it will
important to be able to modify the properties of food proteins. One possibility
use enzymic modifications to improve functional properties and nutritional
A highly efficient class of enzymic crosslinkers are transglutaminases
220.127.116.11), calcium-dependent enzymes that catalyse an acyl transfer reaction
between protein-bound glutaminyl residues and primary amines (for review,
Aeschlimann & Paulsson, 1994). When protein-bound lysyl residues act
acceptors, the reaction leads to the formation of intramolecular and/or
isopeptide bonds. Although a wide variety of amines such as putrescine,
cadaverine or lysyl residues may act as amine donors in this reaction,
only a limited
number of glutamine residues in certain proteins will act as amine acceptors
& Folk, 1980).
Ikura et al. (1981, 1985) reported that TG can be used to introduce
into casein and soyabean proteins and lysine into wheat gluten, thereby
TG can also be utilized to improve the nutritional value of food proteins.
been a number of reports concerning the TG-mediated polymerization of food
proteins such as α-lactalbumin and β-lactoglobulin (Aboumahmoud
& Savello, 1990;
Færgemand et al. 1997), soyabean proteins and casein components
(Ikura et al.
1980a, b) and pea legumin (Larré et al.
1993). Recently, improved protein foaming
capacity and stability have been demonstrated in TG-catalysed polymers
soyabean protein and whey protein isolate (Yildirim et al. 1996).
Bovine PP3 (for review, see Girardet & Linden, 1996) is a phosphorylated
glycoprotein isolated from the proteose peptone fraction of milk (Sørensen
Petersen, 1993a). The primary structure of PP3 has been determined
Petersen, 1993b) and comprises a polypeptide backbone of 135 amino
containing five phosphorylated serines, two threonine-linked O-glycosylations,
one N-glycosylation. Immunological studies have shown that PP3 is present
milk fat globule membrane, and that PP3 forms multimeric aggregates in
milk (Sørensen et al. 1997). Several functions have been
suggested and investigated
for bovine PP3 and fractions enriched with PP3, including emulsification
et al. 1989), inhibition of lipolysis (Girardet et al.
1993) and mitogenesis (Mati et al.
Previously, we have localized the potential TG-reactive glutamines in
bovine caseins (Christensen et al. 1996) and in milk osteopontin
(Sørensen et al. 1994).
In the present study we have shown that component PP3 is a substrate for
guinea-pig liver TG containing both reactive glutamine and lysine residues. In
we have localized the glutamine residues that act as amine acceptors.