INBORN ERROR OF CARBOHYDRATE METABOLISM

Deficiency or absence of an enzyme that participate in carbohydrate metabolism may result in accumulation of monosaccharides, which can be measured in urine. Most of these conditions are inherited as autosomal recessive traits.

DISORDER OF GALACATOSE METABOLISM
Galactose is derived from milk in diet. It is the C4 epimer of glucose. A deficiency of any of the enzyme that participates in conversion of galactose to glucose results in galactosemia. Galactosemia occurs due to inhibition of glycogenolysis.

GALACTOSE-1-PHOSPHATE URIDYL TRANSFERASE DEFICIENCY

Infants with this deficiency fail to thrive on milk because half of the milk sugar, lactose is galactose. Within few days of milk ingestion neonates manifest vomiting and diarrhea. Failure to thrive, liver disease, cataracts and mental retardation develop later. This disorder is identified by measuring erythrocyte galactose -1-phosphate uridyltransferase activity.

GALACTOKINASE DEFICIENCY

This is milder condition manifested by cataracts caused by galactitol deposits in the lens. The diagnosis is confirmed by demonstrating normal transferase activity no galactokinase in red blood cells.

DISORDER OF FRUCTOSE METABOLISM
Fructose may appear in the urine after eating fruits, honey, and syrups, but has no significance in these conditions. Three disorders of fructose metabolism inherited as autosomal recessive trait produces fructosuria.

Essential fructosuria
This occurs due to deficiency of fructokinase

Hereditary fructose intolerance

A deficiency of fructose-1-phosphate aldolase produces this disorder with hypoglycemia and liver failure. Fructose ingestion inhibits glycogenolysis and gluconeogenesis, producing hypoglycemia.

Hereditary fructose-1, 6-diphosphate deficiency

DISORDER OF PENTOSE METABOLISM

Alimentary pentosuria
Pentose may be present in the urine after eating large quantities of fruits such as cherries, plums, or prunes.

Essential pentosuria
This is harmless inborn error caused by deficiency of L-xylulose reductase an enzyme involved in the glucuronic acid pathway.
Individual sugars can be measured by qualitative tests and chromatography