CASE FILES: BIOCHEMISTRY
A N SW E R S TO C A SE 24: F R U C T O SE IN T O L E R A N C E
A 3-year-old boy with failure to thrive and possible hepatic failure.
He presents with hypoglycemia and recurrent episodes of nausea and vomit-
ing after ingestion of foods high in fructose.
Diagnosis: Fructose intolerance.
Biochemical basis of disorder: Because of a genetic disorder, the
hepatic aldolase B enzyme is defective, and functions normally in
glycolysis but not in fructose metabolism. Glucose production is
inhibited by elevated fructose 1-phosphate. When fructose is ingested,
severe hypoglycemia results.
Treatment: Avoid dietary fructose.
C L IN IC A L C O R R E L A T IO N
Individuals with a deficiency in aldolase B have the condition known as fruc-
tose intolerance. As with most enzyme deficiencies, this is an autosomal reces-
sive disease; it does not cause difficulty as long as the patient does not consume
any foods with fructose or sucrose. Frequently, children with fructose intoler-
ance avoid candy and fruit, which should raise some eyebrows! Likewise, they
usually do not have many dental caries. However, if chronically exposed to
fructose-containing foods, infants and small children may have poor weight
gain and abdominal cramping or vomiting.
A PPR O A C H TO D ISA C C H A R ID E M E T A B O L ISM
Know about the metabolism of disaccharides, specifically fructose.
Know about the role of aldolase B in fructose metabolism.
Disaccharide: Two sugar molecules (monosaccharides) linked together by
a glycosidic bond. The major disaccharides obtained in the diet are malt-
-glucose], sucrose [P-
glucopyranoside], and lactose [4-(P-
Essential fructosuria: A rare, benign genetic condition in which fructose
spills over to the urine because the liver, kidney, and intestine lack the
Fructokinase: An enzyme present in the liver, kidney, and intestine that
will phosphorylate fructose to fructose 1-phosphate at the expense of