Food Biochemistry and Food Processing (2 edition)

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26 Equid Milk: Chemistry, Biochemistry and Processing 521

allergenicity of bovine milk; a readjustment of the casein:whey

proteins ratio to 40:60 was found to make the bovine milk sig-

nificantly less allergenic. Presumably, equine and asinine milk,

with a ratio of casein:whey proteins close to that in human milk,

are potentially good substitutes for human milk. It is notewor-

thy that the study by Lara-Villoslada et al. (2005) was carried

out using mice as subjects and the increased level ofβ-Lg (a

major bovine milk allergen) in bovine milk adjusted to 40:60,

casein:whey was not considered.

Cross-Reactivity of Milk Proteins

Cross-reaction occurs when two food proteins have similar

amino acid sequences or when the three-dimensional confor-

mation makes two molecules similar in their capacity to bind

specific antibodies (Restani et al. 2002). Cross-reactivity of pro-

teins from different species generally reflects the phylogenetic

relations between animal species, for example homologous pro-

teins from vertebrates often cross-react. A comprehensive study

on the subject by Jenkins et al. (2007) highlights some interesting

points, especially concerning the potential allergenicity of ca-

seins from different species. The authors set out to determine

how closely a foreign protein has to resemble a human homo-

logue before it actually loses its allergenic affect. A high degree

of similarity to human homologues would, presumably, imply

that a foreign animal food protein would be much less likely than

a protein with little or no similarity to its human homologue to be

allergenic in human subjects. In addition, the study of potential

animal allergens must take into account the ability of the human

immune system to discriminate between its own proteins, that is

an autoimmune response, and those from another species which

have a high similarity, that is how closely a foreign protein has

to resemble a human homologue before it loses its ability to

act as an allergen? (Spitzauer 1999). Table 26.17 gives the per-

centage homology ofαS1-,α-S2andβ-caseins from different

species to bovine and human homologues. Known allergens are

less than 53% identical to human sequences. Natale et al. (2004)

found that 90% of a group of infants with CMPA had serum IgE

against bovineαS2-casein, 55% against bovineαS1-casein and

only 15% against bovineβ-casein, which is closest in amino acid

Table 26.17.Homology (percentage) Between Milk Proteins from Different

Species and Their Human Homologues

Percent Identity to Closest

Casein Accession Code Bovine Homolog Human Homolog

α-S1-Casein

Cow P02662 100 29

Goat Q8M1H4 88 29

Sheep P04653 88 28

Horse Q8SPR1 39 44

Human P47710 29 100

Rat PO2661 22 27

Camel O97943 41 36

Rabbit PO9115 37 37

α-S2-Casein

Cow P02663 100 16

Goat P33049 88 17

Sheep P04654 89 17

Camel O97944 56 11

Rabbit P50418 36 16

β-Casein

Cow P02666 100 53

Goat Q712N8 91 54

Sheep P11839 91 54

Horse Q9GKK3 56 58

Human P05814 53 100

Camel Q9TVD0 66 58

Rabbit PO9116 52 55

Source: Modified from Jenkins et al. 2007.