Agriculture Reference
In-Depth Information
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Sucrose, which is also known as cane or table sugar, is the disaccharide most abun-
dantly consumed in the human diet as natural sweetener. Table sugar is refined from
sugar cane or sugar beets, and various refined forms of sucrose are used in produc-
tion of foods. Sucrose is also found in high concentration in fruits, such as ripe
oranges and apples, where it serves as an easily available reserve of energy for the
fast growth of a new plant. Intestinal digestion of sucrose [α-d-glucopyranosyl(1,2′)
β′-d-fructofuranoside] requires hydrolysis of the α(1,2′)β′ linkage, which involves
the anomeric carbon of both hexoses, to yield glucose and fructose. In mamma-
lian species, the hydrolysis of sucrose is performed by the sucrase activity of the SI
enzyme complex of the epithelial cells lining the intestine. The resulting monosac-
charides are then absorbed by the intestinal epithelial cells into blood circulation.
In contrast to sucrose, isomaltose [
O
-α-d-glucopyranosyl-(1,6)-α-d-gluco-
pyranoside] is a disaccharide found in very low concentrations in natural sources.
However, this disaccharide is assumed to be a limiting product of the enzymatic
hydrolysis of starches. The α-1,6 glucosidic linkage of isomaltose is hydrolyzed by
the isomaltase subunit of the SI enzymatic complex present in the small intestinal
brush border membrane. The reaction releases two molecules of glucose per mole-
cule of isomaltose. Although isomaltose sometimes is considered as a “slow-digest-
ing” product of amylase starch digestion, SI displays about 50% as much isomaltase
activity as its maltase activity, which ensures enough activity to cope with the total
isomaltose that may be produced in the starch digestive process. Although there is
no experimental evidence, probably other α-1,6 branched oligosaccharides, such as
isomaltotriose, isomaltotetrose, and so on, may be responsible for the slow digestion
fraction observed in the hydrolysis products of starch.
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Trehalose [
O
-α-d-glucopyranosyl-(1,1)-α-d-glucopyranoside] is a disaccharide of
minor importance for the contemporary human diet since it is found in appreciable
amounts only in some mushrooms and yeasts as well as in most insects' hemolymph.
In insects, this disaccharide serves as a form of energy mobilization and appears to
contribute to the preservation of the organism's viability after desiccation or expo-
sure to freezing temperatures (Lillford and Holt 2002; Richards et al. 2002; Elbein
et al. 2003). The enzyme responsible for the breakdown of trehalose is trehalase,
which, like SI, LPH, and MGAM, is found in the brush border membrane of the
intestinal epithelial cells; however, instead of the transmembrane peptide domain
that anchors the other three mucosal glucosidic enzymes, trehalase is anchored to
the apical membrane of enterocytes through a phophatidyl-inositol anchor (Semenza
1986). The presence of this enzyme in the human intestinal tract seems odd in view
of the low consumption of foods containing trehalose; in fact, trehalase deficiency is
the least frequently detected anomaly of carbohydrate digestion in humans, probably
due to the low or absent presence of trehalose in modern human diets. However, this
enzyme may have had an important role in early humans, for whom insect and fun-
