Agriculture Reference
In-Depth Information
(Ouwendijk et al. 1996; Ouwendijk et al. 1998; Jacob et al. 2000; Spodsberg et al.
2001a, 2001b; Propsting et al. 2003; Ritz et al. 2003).
m
a L t a s e
-g
L u C o a m y L a s e
The human MGAM gene is located on chromosome 7 (Nichols et al. 2003) and
codes for a protein at least 1857 amino acids long. The studies of the regulation of
MGAM activities have been hampered by the overlapping of substrates and activi-
ties of SI and MGAM. Some studies have suggested that MGAM expression and
activity parallel those of SI (McDonald and Henning 1992). In addition, although
the expression of MGAM mRNA has been observed in diverse tissues, its extra-
intestinal functions have not been described (Pereira and Sivakami 1991; Ben Ali
et al. 1994). Elucidation of MGAM expression patterns has been complicated even
more by the evident amplification of the gene segment coding for the C-terminal
subunit of the protein (Naumoff 2007). In humans, at least four different tandem
C-terminal segment amplifications can be found in the genome, and there is some
evidence indicating the existence of alternative splicing within the primary tran-
script from these amplified segments. Studies of the mechanisms for gene regulation
of this protein await to be performed.
Similar to SI, MGAM is inserted in the plasma membrane by its N-terminal end,
but in contrast to SI, no proteolytic processing has been documented to occur for
human MGAM (Naim et al. 1988). Although single-nucleotide gene polymorphisms
have been described involving changes in the amino acid sequence of the protein, no
association of these changes with deficiencies have been observed.
m
o n o s a C C h a r i D e
t
r a n s P o r t e r s
The transport of monosaccharides through cellular membranes, either at the intes-
tinal epithelial layer or in other body tissues, is mediated by integral membrane
proteins known as glucose transporters (GLTs). At the apical membrane of intestinal
epithelial cells, the transport of glucose and galactose occurs against a concentration
gradient that requires a special transporter able to overcome this energetic barrier
(Wright et al. 1980; Wright et al. 1997, 2003, 2004). This active transporter is called
sodium-dependent glucose transporter 1 (SGLT1) on chromosome 22, which is able
to use the energy produced by the sodium concentration gradient existing across the
cellular membranes to drive the transport of glucose or galactose into the epithelial
cells. In contrast, fructose utilizes GLT5 on chromosome 1, which is a passive trans-
porter that depends on the simple diffusion of this monosaccharide to cross the api-
cal membrane of epithelial cells (Blakemore et al. 1995). Once inside the epithelial
cells, all monosaccharides reach high enough concentration to exit the cell assisted
by a facilitated diffusion transporter called GLT2 located on chromosome 3.
Synthesis of the GLT proteins seems to be directly dependent on transcription of
the respective genes (Corpe and Burant 1996); however, transport of the synthesized
proteins into the apical membrane seems to be dependent on hormonal and neuronal
signals (Shu et al. 1997). While cyclic AMP (cAMP) and protein kinases play impor-
tant roles in the transcription of the respective mRNA coding for the transporters
