Caseins Characteristics in Equid and Human Milk

Equid milk is similar in composition to human milk, even if equids and humans are phylogenetically distantly related. The protein content of milk varies considerably between species and reflects the growth rate of the young. For humans, one of the slowest growing and maturing species, it takes 120-180 days to double birth weight and only 7% of calories come from protein. Equid species take between 30 and 60 days to double their birth weight and, like humans, have an exceptionally low level of protein in their milk. Caseins are the predominant class of proteins in bovine milk, about 80% of total milk protein, while equid and human milk contains less caseins and more whey proteins. The biological function of the caseins lies in their ability to form macromolecular structures, casein micelles, which transfer large amounts of calcium to the neonate with a minimal risk of pathological calcification of the mammary gland. Casein micelles are hydrated spherical structures with dimensions in sub-micron range. Equine micelles are larger than bovine or human micelles, while those of donkey milk are similar in size to bovine micelles. Caseins are precipitated by gastric acid and enzymes, forming a clot in the stomach that entraps fat. The hardness of this clot depends on the casein content of the milk: high casein-containing milks will produce firm clots. Generally, species that nurse their young at frequent intervals, for example equids and humans, tend to produce dilute milk in which less than 60% of total proteins is casein and which form a soft clot, whereas animals that nurse infrequently, such as bovine and ovine, produce milk that is high in fat and caseins and has much longer gastric retention. The contents of casein and whey proteins in human milk change deeply early in lactation: the concentration of whey proteins is very high, whereas casein is virtually undetectable during the first days of lactation. As a consequence, there is not a fixed ratio of whey to casein in human milk, it varies throughout lactation. The major constituent of the family of human caseins is -casein, a highly phosphorilated protein. During digestion, phosphopeptides are formed and have been shown to keep Ca2+ soluble, thus facilitating its absorption. It is therefore likely that phosphopeptides formed from -casein contribute to the high bioavailability of calcium from human milk. K-casein, a minor casein subunit in human milk, is a glycosylated molecule that has been shown to inhibit the adhesion of Helicobacter pylori to human gastric mucosa, by acting as a receptor analogue. The presence of -casein in human milk has not been clearly demonstrated. Protein profile of equid milk is quite similar to human milk, therefore equid milk may be more suitable in human nutrition than bovine milk. In this work the attention was focused especially on the molecular characteristics of caseins from equid and from human milk compared also to those of bovine milk. Furthermore the different allergenic properties of casein from equid and bovine has been taken into account.