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508 Part 4: Milkglycine, alanine and serine are the most abundant free amino
acids in equine, bovine and human milk, and taurine also is ex-
ceptionally high in human milk (Rassin et al. 1978, Sarwar et al.
1998, Carratu et al. 2003). Taurine is an essential metabolite`
for the human infant and may be involved in the structure and
function of retinal photoreceptors (Agostini et al. 2000). Com-
pared to bovine milk, equine milk has an appreciable amount
of taurine although it is ten times less than that of human milk
(Table 26.7). In contrast to total amino acid composition, which
is essentially similar in equine, bovine and human milks, free
amino acids show a pattern characteristic of each species (Table
26.7), which may be important for early post-natal development
in different animals. Free amino acids are more easily absorbed
than protein-derived amino acids and glutamic acid and glu-
tamine, which comprise>50% of the total free amino acids of
human milk, are a source ofα-ketoglutaric acid for the citric
acid cycle and also act as neurotransmitters in the brain (Levy
1998, Agostini et al. 2000).Bioactive PeptidesBoth caseins and whey proteins are believed to contribute to
human health through latent biological activity produced en-
zymatically during digestion, fermentation with specific starter
cultures or enzymatic hydrolysis by microorganisms, resulting
in the formation of bioactive peptides. These peptides are im-
portant for their physiological roles, their opioid-like features,
as well as their immunostimulating and anti-hypertensive activi-
ties and their ability to enhance Ca^2 +absorption and are released
or activated during gastrointestinal digestion. Several peptides
generated by the hydrolysis of milk proteins are known to reg-
ulate the overall immune function of the neonate (Baldi et al.
2005). A detailed discussion on bioactive peptides in milk is out-
side the scope of this chapter, for reviews, see Donnet-Hughes
et al. (2000), Shah (2000), Malkoski et al. (2001), Fitzgerald
and Meisel (2003), Silva and Malcata (2005), Fitzgerald and
Murray (2006), Lop ́ez-Fandino et al. (2006), Michaelidou and ̃
Steijns (2006), Thoma-Worringer et al. (2006) and Phelan et al. ̈
(2009).
Research on the bioactive peptides derived from equid milk
is very limited. Peptides from the hydrolysis of equineβ-
casein may have a positive action on human health (Doreau
and Martin-Rosset 2002). Chen et al. (2010) reported the pres-
ence of four novel angiotensin-converting enzyme-inhibitory
peptides in koumiss which may enhance the beneficial effects
of koumiss on cardiovascular health. Peptides with trophic or
protective activity have been identified in asinine milk (Salimei
2011).Hormones and Growth FactorsLeptin is a protein hormone of approximately 16kDa that has
been discovered recently in human milk and plays a key role
in the regulation of energy intake and energy expenditure, as
well as functioning in mammary cell proliferation, differentia-
tion and apoptosis. Human-like leptin has been isolated from
asinine milk at a level of 3.2–5.4 ng.mL−^1 which is similar tolevels reported for other mammals and showed little variation
throughout lactation (Salimei et al. 2002).
Levels of the bioactive peptides, ghrelin and insulin growth
factor I, which play a direct role in metabolism, body composi-
tion and food intake, have also been reported for asinine milk at
4.5 pg.mL−^1 and 11.5 ng.mL−^1 , respectively, similar to levels
in human milk (Salimei 2011).Amyloid AAmyloid A3 (AA3) is a protein produced in the mammary gland
and is encoded by a separate gene from that for serum amyloid
A (serum AA) (Duggan et al. 2008). AA3 is believed to pre-
vent attachment of pathogenic bacteria to the intestinal cell wall
(Mack et al. 2003) and may prevent necrotising enterocolitis
in human infants (Larson et al. 2003). McDonald et al. (2001)
demonstrated the presence of AA3 in the colostrum of cows,
ewes, sows and horses. Bovine colostrum has a high concentra-
tion of AA3 but by approximately 3 dayspost-partumthe levels
decline. In bovine milk, the presence of serum AA in milk is an
indicator of mastitic infection (Kaneko et al. 2004, Winter et al.
2006). In equine colostrum, the concentration of AA3 is consid-
erably lower than in milk and consequently may play a crucial
role in intestinal cell protection in the foal especially after gut
closure (Duggan et al. 2008).INDIGENOUS ENZYMES
Milk contains many indigenous enzymes that originate from
the mammal’s blood plasma, leucocytes (somatic cells), or cy-
toplasm of the secretory cells and the milk fat globule mem-
brane (MFGM)(Fox and Kelly 2006). The indigenous enzymes
in bovine and human milks have been studied extensively but
the enzymes in the milk of other species have been studied only
sporadically. Equine milk probably contains all the enzymes that
have been identified in bovine milk but relatively few studies
have been reported.LysozymeLyz (EC 3.1.2.17) occurs at high levels in equine, asinine and
human milk (Table 26.2). Human, equine and asinine milk con-
tain 3000, 6000 and>6000 times more Lyz, respectively, than
bovine milk (Salimei et al. 2004, Guo et al. 2007) with levels
as high as 0.4 g.100g−^1 for Martina Franca donkeys (Coppola
et al. 2002) although 0.1 g/100 g is reported and more commonly
found in asinine milk (Vincenzetti et al. 2008). The concentra-
tion of Lyz in human milk increases strongly after the second
month of lactation, suggesting that Lyz and Lf play major roles
in fighting infection in breast-fed infants during late lactation,
and protect the mammary gland (Montagne et al. 1998).
Equine milk Lyz is more stable to denaturation than human
Lyz during pasteurisation at 62◦C for 30 minutes but at 71◦Cfor
2 minutes or 82◦C for 15 seconds, the inactivation of both were
similar (Jauregui-Adell 1975). It has been suggested, but re-
search is scarce, that while the composition of breast milk varies