Milk Oligosaccharides - Advanced Dairy Chemistry - page 323

(2005) formulated the hypothesis that the LNB structure which occurs at the
non-reducing end of several human milk oligosaccharides is a specific bifidus
factor (Figure 8.3). Since LNB-containing oligosaccharides predominate over
those containing N-acetyllactosamine in human milk (see above), this LNB
hypothesis seems attractive.
It should be noted that free LNB is not found among the human milk
oligosaccharides. For the hypothesis to be valid, bifidobacteria would have to
have enzymes that liberate LNB from human milk oligosaccharides, since
LNBP cannot act on -glycosides of LNB (Derensy-Dron et al., 1999; Kitaoka
et al., 2005). Considering the fact that lacto-N-tetraose and lacto-N-fuco-
pentaose I are the major components of human milk oligosaccharides along
with 2 0 -FL, lacto-N-biosidase and -fucosidase enzymes would be required for
the entry of these oligosaccharides into the LNB pathway (Figure 8.3). The
gene encoding an -fucosidase that hydrolyzes lacto-N-fucopentaose I and 2 0 -
FL has been already cloned from B. bifidum (Katayama et al., 2004). This
activity was found in the culture supernatant of B. bifidum but not of B. breve or
B. longum. As a lacto-N-biosidase that hydrolyzes lacto-N-tetraose into lacto-
N-biose I and lactose has recently been cloned from B. bifidum JCM1254

α -fucosidase

LNFP I

β 1-3Gal β 1-4Glc

Fuc α 1-2 Gal β 1-3GlcNAc

lacto-N-biosidase

β 1-3Gal β 1-4Glc Gal β 1-3GlcNAc

LNT

transport of LNB into cell

LNB specific pathway

Bifidobacterium cell

Figure 8.3. The lacto-N-biose hypothesis. Bifidobacteria secrete hydrolytic enzymes that act on
milk oligosaccharides to form lacto-N-biose, which is selectively transported into bifidobacterial
cells to be metabolized by lacto-N-biose phosphorylase.