Glycogen storage disease
All mammalian cells can manufacture glycogen, but the main sites of its production are the liver and muscle. Glycogen is a high-molecular-weight glucose polymer. In glycogen storage disease there is either an abnormality in the molecular structure or an increase in glycogen concentration owing to a specific enzyme defect. Almost all these conditions are autosomal recessive in inheritance and present in infancy, except for McArdle’s disease, which presents in adults .The classification and clinical features of some of these diseases. New specific enzyme defects, e.g. liver phosphorylase, phosphorylase kinase, are being recognized.
Galactose is normally converted to glucose. However, a deficiency of the enzyme galactose-j-phosphate uridyltransferase results in accumulation of galactose-I-phosphate in the blood. This deficiency, inherited as an autosomal recessive, results in hypoglycaemia and acidosis in the neonate. Progressive hepatosplenomegaly, cataracts,
renal tubular defects and mental retardation occur. Treatment is with a galactose-free diet, which, if started early, results in normal development. Untreated patients die within a few days. Prenatal diagnosis and diagnosis of the carrier state are possible by measurement of the level of galactose-I-phosphate in the blood.
Galactokinase deficiency also results in galactosaemia and early cataract formation.
Defects of fructose metabolism
Absorbed fructose is chiefly metabolised in the liver to lactic acid or glucose. Three defects of metabolism occur;all are inherited as autosomal recessive traits:
FRUCTOSURIA is due to fructokinase deficiency. It is a benign condition.
FRUCTOSE INTOLERANCE is due to fructose-lphosphate aldolase deficiency. Fructose-J-phosphate accumulates after fructose ingestion, resulting in symptoms of hypoglycaemia. Hepatomegaly and renal tubular defects occur but are reversible on a fructose-free diet. Intelligence is normal and there is an absence of dental caries.
FRUCTOSE-l ,6-DIPHOSPHATE DEFICIENCY leads to a failure of gluconeogenesis, and to hepatomegaly.
Pentosuria is due to L-xylulose reductase deficiency. It has no clinical significance.