CLINICAL CASES
219
Fructose intolerance: A genetic deficiency in the liver enzyme aldolase B.
The absence of this enzyme leads to a build up of fructose 1-phosphate
and depletion of liver ATP and phosphate stores.
GLUT 5: A facilitative glucose transporter isoform present in the small
intestine and other tissues that will transport fructose (and glucose to a
lesser extent) across the plasma membrane.
b-Glycosidase: A bifunctional, membrane-bound enzyme located on the
brush-border membrane of the small intestine. This single polypeptide
enzyme has two activities, lactase and glycosylceramidase, located in
different domains of the protein. It will hydrolyze lactose to glucose and
galactose.
SGLT1: A sodium-dependent glucose transporter located on the luminal
side of the intestinal epithelial cells. It will transport glucose and galac-
tose across the intestinal cell using a sodium ion gradient.
SGLT2: A sodium-dependent glucose transporter that has a high specificity
for glucose and is specific to the kidney.
Sucrase-isomaltase complex: An enzyme complex comprised of two
enzyme units. Both units have high a-1,4-glucosidase activity and will
hydrolyze maltose and maltotriose to glucose. The sucrase unit will also
hydrolyze sucrose to fructose and glucose, whereas the isomaltase unit
will hydrolyze a-1,6 bonds found in isomaltose and the limit dextrins of
starch.
D ISC U SSIO N
The major disaccharides obtained in the diet are maltose, sucrose, and lac-
tose. Maltose is primarily obtained from the consumption of the plant stor-
age polysaccharide starch. Starch is degraded to glucose and small branched
oligosaccharides called limit dextrins by exhaustive digestion by a-amylase.
The limit dextrins are further enzymatically hydrolyzed to a branched tetrasac-
charide by glucoamylase and to maltotriose and maltose and ultimately to glu-
cose by the sucrase-isomaltase complex. Both of these enzyme complexes are
located on the brush-border membrane of the small intestine. Sucrose, or
table sugar, is hydrolyzed to glucose and fructose by the sucrase subunit of
the sucrase-isomaltase complex. Lactose, or milk sugar, is enzymatically con-
verted to glucose and galactose by b-glycosidase, also located on the brush-
border membrane of the small intestine. This membrane-bound enzyme is a
single polypeptide that has lactase and glycosylceramidase activities located in
different domains of the protein.
Glucose and galactose are absorbed from the lumen of the intestine by the
sodium-dependent glucose transporter, SGLT1, which is located on the
luminal side of the intestinal epithelial cells. Fructose absorption is not
dependent on a sodium gradient. It is transported into the intestinal cell by
facilitative diffusion by a glucose transporter isoform, GLUT 5. Fructose
transport is less rapid than glucose transport, and GLUT 5 does not have a high
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