Food Biochemistry and Food Processing (2 edition)

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458 Part 4: Milk

caseins, PP3 is an indigenous milk protein, synthesised in the

mammary gland. Bovine PP3 is a polypeptide of 135 amino

acids, with five phosphorylation and three glycosylation sites.

When isolated from milk, the PP3 fraction contains at least

three components of MW∼28, 18 and 11 kDa; the largest

of these is PP3, while the smaller components are fragments

thereof generated by plasmin (see Girardet and Linden 1996).

PP3 is present mainly in acid whey but some is present in the

MFGM also. Girardet and Linden (1996) proposed that its name

should be changed tolactophorinorlactoglycoporin. PP3 has

also been referred to asthe hydrophobic fraction of proteose

peptone.

Owing to its strong surfactant properties (Campagna et al.

1998), PP3 can prevent contact between milk lipase and its

substrates, thus preventing spontaneous lipolysis. Although its

amino acid composition suggests that PP3 is not a hydrophobic

protein, it behaves hydrophobically, possibly owing to the for-

mation of an amphiphilicα-helix, one side of which contains

hydrophilic residues while the other side is hydrophobic. The

biological role of PP3 is unknown.

Vitamin-Binding Proteins

Milk contains binding proteins for at least the following vi-

tamins: retinol (vitamin A, i.e.,β-Lg, see Section ‘Molecular

Properties of Milk Proteins’), biotin, folic acid and cobalamine

(vitamin B 12 ). The precise role of these proteins is not clear but

they may improve the absorption of vitamins from the intestine

or act as antibacterial agents by rendering vitamins unavailable

to bacteria. The concentration of these proteins varies during

lactation, but the influence of other factors such as individu-

ality, breed and nutritional status is not known. The activity of

these proteins is reduced or destroyed on heating at temperatures

somewhat higher than HTST pasteurisation.

Angiogenins

Angiogenins induce the growth of new blood vessels, that is

angiogenesis. They have high sequence homology with mem-

bers of the RNase A superfamily of proteins and have RNase

activity. Angiogenesis is a complex biological process of which

the ribonuclease activity of angiogenins is one of a number of

essential biochemical steps that lead to the formation of new

blood vessels (Strydom 1998).

Two angiogenins (ANG-1 and ANG-2) have been identified

in bovine milk and blood serum. Both strongly promote the

growth of new blood vessels in a chicken membrane assay.

Bovine ANG-1 has 64% sequence identity with human angio-

genin and 34% identity with bovine RNase A. The amino acid

sequence of bovine ANG-2 has 57% identity with that of bovine

ANG-1; ANG-2 has lower RNase activity than ANG-1. The

function(s) of the angiogenins in milk is unknown; they may

be part of a repair system to protect either the mammary gland

or the intestine of the neonate and/or part of the host-defence

system.

Kininogen

Two forms of kininogen have been identified in bovine milk, a

high (>68 kDa) and a low (16–17 kDa) MW form (Wilson et al.

1989). Bradykinin, a biologically-active peptide containing nine

amino acids that is released from the high MW kininogen by

the action of the enzyme, kallikrein, has been detected in the

mammary gland, and is secreted into milk, from which it has

been isolated. Plasma kininogen is an inhibitor of thiol proteases

and has an important role in blood coagulation. Bradykinin af-

fects smooth muscle contraction, induces hypertension and is

involved in natriuresis and diuresis. The biological significance

of bradykinin and kininogen in milk is unknown.

Glycoproteins

Many of the minor proteins discussed previously are glycopro-

teins; in addition, several other minor glycoproteins have been

found in milk and colostrum but their identity and function have

not been elucidated fully. Some of these glycoproteins belong to

a family of closely related, highly acidic glycoproteins, called

M-1 glycoproteins. Some glycoproteins stimulate the growth of

bifidobacteria, presumablyviatheir amino sugars. One of the M-

1 glycoproteins in colostrum, but which has not been detected

in milk, is orosomucoid (α 1 -acid glycoprotein), a member of

the lipocalin family, which is thought to modulate the immune

system.

One of the high MW glycoproteins in bovine milk is pros-

aposin, a neurotrophic factor that plays an important role in

the development, repair and maintenance of the nervous system

(Patton et al. 1997). It is a precursor of saposins A, B, C and

D, which are sphingolipid-activator proteins, but saposins have

not been detected in milk. The physiological role of prosaposin

in milk is not known, although the potent biological activity

of saposin C, released by digestion, could be important for the

growth and development of the young.

Proteins in the Milk Fat Globule Membrane

About 1% of the total protein in milk is in the MFGM. Most of the

proteins are present at trace levels, including many of the indige-

nous enzymes in milk. The principal proteins in the MFGM in-

clude MUC1 and MUC15, adipophilin, BTN and XDH (Keenan

and Mather 2003, 2003, 2006). BTN is a very hydrophobic pro-

tein and has similarities to the Igs (for review see Mather 2000,

2011).

Growth Factors

A great diversity of protein growth factors (hormones), including

epidermal growth factor, insulin, insulin-like growth factors 1

and 2, three human milk growth factors (α 1 ,α 2 andβ), two

mammary-derived growth factors (I and II), colony-stimulating

factor, nerve growth factor, platelet-derived growth factor and

bombasin, are present in milk. It is not clear whether these

factors play a role in the development of the neonate or in the