Chemistry Reference
In-Depth Information
Since lactose is an obligatory acceptor for the mammary glycosyltrans-
ferases,
-lactalbumin is also essential for the presence of milk oligosaccharides.
The exact number of glycosyltransferases involved in the biosynthesis of the
oligosaccharides is still uncertain. These enzymes are known to be very specific,
their specificity being directed toward both the type of linkage and the acceptor
molecule. For example, in addition to the above-mentioned
-galactosyltrans-
ferase, there are probably at least two other human mammary
-galactosyl-
transferases, the actions of which are independent of
-lactalbumin, that cata-
lyze the synthesis of Gal(
1-3)GlcNAc-R and Gal(
1-4)GlcNAc-R structures
such as those shown in Table 8.1. Although human milk contains traces of three
different galactosyllactoses, it is possible that these trisaccharides are formed by
the transferase actions of
-galactosidase rather than by three specific galacto-
syltransferases. Judging from the variety of structures among the human milk
oligosaccharides (Table 8.1) one can assume that the human mammary gland
probably contains at least three
-N-acetylglucosaminyltransferases, three
-fucosyltransferases and two sialyltransferases.
Additional glycosyltransferases are found in lactating mammary glands
of non-human species. An
-galactosyltransferase that synthesizes
-3
0
-galac-
tosyllactose (isoglobotriose), a trisaccharide present in bovine, ovine and capr-
ine colostrum and the milk or colostrum of several other species but not in
humanmilkorcolostrum(Urashimaet al., 2001). The synthesis of
-4
0
-galac-
tosyllactose (globotriose), which has been found in colostrum of the bottle-
nosed dolphin (Uemura et al., 2005), is presumably catalyzed by a different
-galactosyltransferase. Lactating mammary glands of the tammar wallaby
contain a very active
-galactosyltransferase that is involved in the synthesis
of a series of
(1-3)-linked galactosyllactoses that are unique to the milk of
marsupials (Messer and Nicholas, 1991). The mammary glands of this species
also contain an unusual
-N-acetylglucosaminyltransferase that attaches a N-
acetylglucosaminyl residue to the trisaccharide
-3
0
-galactosyllactose and to the
tetrasaccharide
-3
0
,3
00
-digalactosyllactose (Urashima et al., 1992).
Although the milk/colostrum of most mammalian species contains lac-
tose as the dominant saccharide, constituting more than 80% of the carbohy-
drate, milk oligosaccharides are found at higher concentrations than lactose in
the milk of monotremes, marsupials and a few species of eutherians such as
bears, giant panda, mink and white-nosed coati (Urashima et al., 2001; Messer
and Urashima, 2002). It has been suggested that the ratio of oligosaccharides to
lactose in milk is based on the ratio of expression of the glycosyltransferases to
-lactalbumin within the lactating mammary gland (Messer and Urashima,
2002). In those species whose mammary glands are characterized by a low
expression of
-lactalbumin, the biosynthesis of lactose is likely to be relatively
slow; therefore, their mammary glycosyltransferases would tend to utilize
almost all the available free lactose for the synthesis of oligosaccharides. In
