MILK PROTEINS^23 14.16.5 Growth factors
The term ‘growth factor’ is applied to a group of potent hormone-like
polypeptides which play a critical role in the regulation and differentiation
of a variety of cells acting through cell membrane receptors. The milk and,
especially, colostrum from several species contain several growth factors,
including insulin-like growth factors (IGF1, IFG2), transforming growth
factors (TGF,,, TGF,,, TGF,), mammary-derived growth factors (MDGF
I, MDGF II), fibroblast growth factors, platelet-derived growth factor
(PDGF) and bombasin.
The source of these polypeptides may be blood plasma, mammary gland
or both. The biological significance of these growth-promoting activities in
colostrum and mature milks is not yet clear. In terms of possible physiologi-
cal significance, two potential targets may be considered, i.e. the mammary
gland or the neonate. In general, most attention has focused on the latter.
It is not known whether the factors in milk that possess the capacity to
promote cell proliferation (1) influence growth of mammary tissue, (2)
promote the growth of cells within the intestines of the recipient neonate, or
(3) are absorbed in a biologically active form and exert an effect on enteric
or other target organs.
Methods using ultrafiltration and chromatography have been developed
for the concentration of growth factors from whey. In addition to possible
food (nutritceutical) applications for such growth factors, a major potential
application is in tissue cultures, for which foetal bovine serum is used as a
source of growth factors. However, the supply of foetal bovine serum is
limited, unreliable, expensive and of variable quality. Whey-derived growth
factors have the potential to have a major impact on the biotechnological
and pharmaceutical industries for the production of vaccines, hormones,
drugs, monoclonal antibodies, and the production of tissue, especially skin
for treatment of burns, ulcers and lacerations.
4.16.6 BiJidus factors
Special types of growth factors are those that promote the growth of
bifidobacteria. It has been recognized for many years that breast-fed babies
are more resistant to gastroenteritis than bottle-fed babies. This is un-
doubtedly a multifactorial phemonenon, including better hygiene, more
appropriate milk composition, several antibacterial systems (especially im-
munoglobulins, lysozyme, lactotransferrin, vitamin-binding proteins and
lactoperoxidase, which are discussed above), and a lower intestinal pH. The
mean pH of the faeces of breast-fed babies is 5.1 while that of bottle-fed
babies is 6.4; the low pH of the former may be due partly to the lower
buffering capacity of human milk compared to bovine milk, due to its lower
content of protein and phosphate, and partly to differences in the intestinal
microflora of breast-fed and bottle-fed infants. Bifidobacteria represent