Disorders of Fructose Metabolism

There are three disorders of fructose metabolism, all inherited in an autosomal recessive fashion. Fructose is widely distributed in the diet as the primary sugar in fruits, vegetables, and honey. It is also derived from sucrose and sorbitol, which are found in large variety of products, including infant formulas and intravenous fluids. The toxic effect of fructose is due to inhibition

Table 3 Glycogen storage disorders

Disorder

Deficient enzyme

Primary affected tissue Symptoms

Treatment

GSD O

GSD I

Glycogen synthase

Liver

Glucose-6-phosphatase Liver, muscle

GSD II (Pompe disease)

GSD III

Acid maltase (a glucosidase)

Debranching enzyme (amylo-1, 6-glucosidase)

GSD IV (amylopectinosis) Branching enzyme

GSD V (McArdle disease) Myophosphorylase

GSD VI (Hers disease) Liver phosphorylase

GSD VII (Tarui disease) Phosphofructokinase

GSD IX

Phosphorylase kinase

Lysosomes of muscle (skeletal and cardiac) Liver, muscle

Liver

Muscle

Liver

Muscle

Liver, muscle

Hypoglycemia

Hypoglycemia, hepatomegaly, growth retardation, proteinuria, lactic acidemia, hyperlipidemia, hyperuricemia (gout), hepatocellular carcinoma Cardiomyopathy, skeletal myopathy, cardiorespiratory failure Hypoglycemia (mild), hepatomegaly, myopathy, hyperlipidemia Hepatomegaly, cirrhosis, liver failure, myopathy

Muscle cramping (with exercise) Hepatomegaly, hypoglycemia, myopathy Fatigue exercise intolerance, cramping

Hepatomegaly, growth retardation

Uncooked cornstarch, frequent feeds Uncooked cornstarch, frequent feeds

Enzyme replacement (in clinical trial) Uncooked cornstarch, frequent feeds

Liver transplant, Uncooked cornstarch Oral glucose, high-protein diet Frequent feeds

Avoidance of strenuous exercise Frequent feeds

GSD, glycogen storage disorder.

of gluconeogenesis by high levels of fructose-1-phosphate and subsequent depletion of inorganic phosphate and, thus, adenosine triphosphate.

Essential fructosuria is a benign disorder due to a defect of the enzyme fructokinase. Patients have increased urinary excretion of fructose, which is usually an incidental finding on routine testing for reducing substances.

Hereditary fructose intolerance is due to a deficiency in aldolase B, which splits fructose-1-phosphate into glyceraldehyde and dihydroxyacetone. Symptoms occur only after exposure to fructose, usually from dietary ingestion although they are more severe after intravenous infusion. The symptoms include gastrointestinal discomfort, vomiting, and hypoglycemia. Chronic exposure causes failure-to-thrive, liver disease, and renal tubular acidosis. Affected patients are often misdiagnosed as having behavioral problems or an eating disorder. Treatment involves elimination of fructose from the diet.

Fructose-1,6-bisphosphatase deficiency is a defect of gluconeogenesis and not dependent on exposure to fructose. Symptoms, including recurrent episodes of vomiting, lactic acidosis, tachypnea, seizures, and apnea, occur when dietary glucose and glycogen stores are depleted, for example during periods of fasting or febrile illness. Approximately 50% of patients are symptomatic in the newborn period. Treatment involves the prevention of fasting and supplementation with uncooked cornstarch to provide a source of complex carbohydrate. Acute episodes respond to intravenous infusions of dextrose.

Abbreviations

ASL

Argininosuccinate lyase

ASS

Argininosuccinate synthetase

BCAA

Branched chain amino-acids

BH4

Tetrahydrobiopterin

BIA

Bacterial inhibition assay

CPS

Carbamoyl synthetase

CPT

Carnitine palmityl transferase

GALT

Galactose-1-phosphate uridyl transferase

GSD

Glycogen storage disorder

HCS

Holocarboxylase synthetase

LCHAD

Long-chain 3-hydroxy acyl CoA

dehydrogenase

MCAD

Medium-chain acyl CoA dehydrogenase

MS/MS

Tandem mass spectrometry

MSUD

Maple syrup urine disease

OTC

Ornithine transcarbamylase

PAH Phenylalanine hydroxylase

PKU Phenylketonuria

SCAD Short-chain acyl CoA dehydrogenase

VLCAD Very long-chain acyl CoA dehydrogenase

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