Agriculture Reference
In-Depth Information
galactose-1P is exchanged for the glucose group present in UDP-glucose by the
enzyme galactose-1P uridine transferase, releasing UDP-galactose and glucose-1P.
UDP-galactose is then transformed into UDP-glucose by UDP-galactose 4-epi-
meraze. Deficiency of galactose-1P uridine transferase is an important form of
galactose intolerance, causing jaundice, cirrhosis, cataracts, and CNS malfunction,
probably as a consequence of intracellular accumulation of galactose. Once trans-
formed into UDP-glucose, the monosaccharide can follow the glucogenic pathway or
can be transformed into glucose-1P to follow any of the regular catabolic pathways
of glucose (Mathews et al. 1999).
RegulAtIon of enzyme And tRAnsPoRt
PRoteIn synthesIs And PRocessIng
a
m y L a s e
Salivary and pancreatic amylases are coded by the genes AMY1 and AMY2, respec-
tively, located in the human chromosome 1. In the typical human haploid genome,
two copies of AMY2 (AMY2 A and B) are present (Zabel et al. 1983). However, the
presence of multiple copies of the segment containing both AMY1 and AMY2 genes
is common among human normal populations. In fact, an association between the
amounts of starch ingested by selected human ethnic groups and the frequency of
multiple copies of the genes has been observed, suggesting that multiple copies of
amylase genes may be an adaptive feature for efficient digestion of high-starch diets
(Iafrate et al. 2004).
Amylase secretion during early postnatal development is usually low, probably
reflecting the still-immature pancreas (Montgomery et al. 1991). After birth, full
production of amylases occurs as a weaning adaptation to carbohydrate feeding,
and this stimulus remains during life as the primary stimulus leading to the synthe-
sis, processing, and secretion of pancreatic amylase and other digestive enzymes.
Thus, signaling for production and release of amylases seems to be mediated by
neuronal pathways. Several neuroendocrine mediators, such as cholecystokinin or
acetylcholine, have been observed to induce the secretion of zymogen granules from
exocrine pancreatic cells via a Ca
2+
influx-mediated mechanism (Williams 2001;
Wasle and Edwardson 2002; Williams et al. 2002). Although the levels of expression
of the messenger RNA (mRNA) coding for pancreatic amylase can be modified, the
protein synthesis seems to be the actual limiting step controlling the production of
pancreatic amylase. Salivary and pancreatic amylases are synthesized as proteins
that are 511 amino acids long with molecular weight near 78 kDa. After cleavage of
a secretory peptide 15 amino acids long, the mature form is transported and stored
into secretory granules. The enzymes require Ca
2+
and Cl
−
ions to display their full
activity (Brayer et al. 2000; Maurus et al. 2005).
L
a C t a s e
-P
h L o r i z i n a s e
The gene coding for human lactase-phlorizin hydrolase is located on chromosome
2 (Kruse et al. 1988). Lactase-phlorizin hydrolase is synthesized as a polypeptide of
