Abstract
Many human milk proteins are glycosylated. Glycosylation is important in protecting bioactive proteins and peptide fragments from digestion. Protein-linked glycans have a variety of functions; however, there is a paucity of information on protein-linked glycan degradation in either the infant or the adult digestive system. Human digestive enzymes can break down dietary disaccharides and starches, but most of the digestive enzymes required for complex protein-linked glycan degradation are absent from both human digestive secretions and the external brush border membrane of the intestinal lining. Indeed, complex carbohydrates remain intact throughout their transit through the stomach and small intestine, and are undegraded by in vitro incubation with either adult pancreatic secretions or intact intestinal brush border membranes. Human gastrointestinal bacteria, however, produce a wide variety of glycosidases with regio- and anomeric specificities matching those of protein-linked glycan structures. These bacteria degrade a wide array of complex carbohydrates including various protein-linked glycans. That bacteria possess glycan degradation capabilities, whereas the human digestive system, perse, does not, suggests that most dietary protein-linked glycan breakdown will be of bacterial origin. In addition to providing a food source for specific bacteria in the colon, protein-linked glycans from human milk may act as decoys for pathogenic bacteria to prevent invasion and infection of the host. The composition of the intestinal microbiome may be particularly important in the most vulnerable humans-the elderly, the immunocompromised, and infants (particularly premature infants).
Highlights
Many proteins in human milk are glycosylated, including lactoferrin, mucin 4, αlactalbumin, lactadherin, κ-casein, butyrophilin, lactoperoxidase, xanthine oxidase, bile saltstimulated lipase, α-1-antichymotrypsin, α-1-antitrypsin, a variety of immuglobulins, and at least 26 other proteins [1,2,3]
Undigested materials in the distal gastrointestinal tract (GIT) of humans include polysaccharides from plant cell walls, undigested starch [23], human milk oligosaccharides (HMO) [31], and host-derived mucin-linked glycans and glycosphingolipids [23]. As these complex carbohydrates and human milk Protein-linked glycans (PLG) have many of the same glycosidic bond structures, human PLG likely survive intact to the large intestine, though this has not been demonstrated
Though little HMO digestion occurs in the upper intestine, extensive hydrolysis likely occurs in the lower intestine/colon, as only about 8% of ingested HMO are found in the infant feces [110]
Summary
Many proteins in human milk are glycosylated, including lactoferrin, mucin 4, αlactalbumin, lactadherin, κ-casein, butyrophilin, lactoperoxidase, xanthine oxidase, bile saltstimulated lipase, α-1-antichymotrypsin, α-1-antitrypsin, a variety of immuglobulins, and at least 26 other proteins [1,2,3]. The various monosaccharides comprising PLG are linked in a variety of positions and with a variety of linkage types, each of which requires a specific glycosidase ( known as glycoside hydrolases) for cleavage Both adults and infants produce enzymes capable of hydrolyzing disaccharides and starch; no studies have demonstrated secreted or external brush border glycosidases specific for the bond types present in human milk PLG. Longum DJO10A, and B. breve ATCC 15700 [28], but do not support growth of bacteria such as Enterococcus faecalis, Streptococcus thermophilus, Veillonella parvula, Eubacterium rectale, Clostridium difficile, and Escherichia coli [22] This degree of specific promotion of bacteria by HMO suggests a similar role for milk PLG. The structures of a relatively low number of human milk PLG are characterized to date, Table 1 is likely not an exhaustive list
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