Sulfite oxidase deficiency is characterized by severe neurologic dysfunction, dislocation of the lenses, and the accumulation and excretion of inorganic sulfite, thiosulfate, and S-sulfocysteine. We present the clinical, radiologic, and biochemical findings in two patients with this condition. In both, neurologic problems started soon after birth and progressed rapidly to profound mental retardation, microcephaly, blindness, and spastic quadriparesis. Seizures were a persistent problem throughout the course of their illness. The neurologic abnormalities were associated with progressive destruction of brain tissue. We established the diagnosis of sulfite oxidase deficiency by demonstrating the characteristic abnormal metabolites in urine. However, commonly used screening procedures do not detect these compounds, and dislocation of the lenses is usually a late feature of the disease. As a result, the diagnosis may be easily overlooked, especially during infancy. Specific investigations for sulfite oxidase deficiency are indicated for any baby with severe, progressive neurologic disease.
A second child with a more severe deficiency of malonyl CoA decarboxylase is described. He is mildly mentally retarded and presented with vomiting, a seizure, hypoglycaemia and mild metabolic acidosis during a urinary tract infection. The urine contained increased amounts of malonic, methylmalonic, succinic, adipic, glutaric and suberic acids. Mitochondrial malonyl CoA decarboxylase activity in cultured fibroblast extracts was 4% of the mean control value. A high fat, low carbohydrate diet led to symptomatic hypoglycaemia, a moderate metabolic acidosis and excretion in the urine of large amounts of the same organic acids and 3-hydroxybutyrate. Only relatively small quantities of malonic, methylmalonic and succinic acid were excreted in the urine when the boy was fed an isocaloric low fat, high carbohydrate diet. Acute fat and lysine loads led to increased excretion of malonic acid in the urine without affecting the excretion of the other organic acids. Experience with this patient suggests that malonyl CoA decarboxylase serves an important function in the mitochondrion by preventing accumulation of malonyl CoA. The importance of the enzyme is best seen when fat is the main metabolic fuel. The mechanisms by which malonyl CoA produces its complex metabolic effects remain to be elucidated.
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