Urocanase deficiency
Incidence
It is a very rare autosomal recessive disorder that appears to be benign. There has been ten reported cases.
Clinical Characteristics
The main symptoms are mental retardation or developmental delay that can be present in infancy and childhood. Patients may have severe mental retardation, episodic aggression or exagerated affection-seeking behavior, short stature, blond hair, blue eyes. The defective enzyme urocanase is normally expressed in liver. The block in conversion of urocanic acid to imidazolone propionic acid results in greatly increased concentration of urocanic acid in urine. Histidine and its metabolites are normal or only mildly increased. Increased urinary excretion or urocanic acid is diagnostic of urocanase deficiency. Histidine loading will exaggerate urocanic acid excretion and also lead to the production of imidazole propionic acid, a biproduct of urocanic acid. Imidazonole priopionic acid and formimino glutamic acid which are distal to the metabolic step catalyzed by urocanase are not present in urine after loading with histidine. The diagnosis of urocanase deficiency is confirmed by demonstrating the absence or marked reduction of urocanase activity in liver. Ketones may be present in urine, tested by ferric chloride test. Urocanic acid is elevated in urine at all ages.
Precipitants
no
Provocation Tests
In normal children, histidine loading will exaggerate urocanic acid excretion and also lead to the production of imidazole propionic acid, a biproduct of urocanic acid. Imidazonole priopionic acid and formimino glutamic acid which are distal to the metabolic step catalyzed by urocanase are not present in urine after loading with histidine.
Diagnostic Procedures
Increased urinary excretion or urocanic acid is diagnostic of urocanase deficiency. Urocanic acid is elevated in urine at all ages.