Preferred Minimum: 1.0 mL urine Absolute Minimum: 0.3 mL urine

Molybdenum cofactor deficiency and isolated sulfite oxidase deficiency are rare inborn errors of metabolism. Molybdenum cofactor is essential for the function of the human enzymes sulfite oxidase, xanthine dehydrogenase and aldehyde oxidase. In patients with either molybdenum cofactor deficiency or isolated sulfite oxidase deficiency, clinical symptoms can vary in severity and age of onset. The characteristic presentation includes severe seizures, often refractory to anticonvulsants, and other neurological abnormalities. Patients who survive the neonatal period may develop dislocated lenses providing an additional and more specific diagnostic clue. S-sulfocysteine is a disease-specific metabolite that accumulates in patients with both forms of the deficiency. Determination of this metabolite in urine by our method provides the most reliable indication of molybdenum cofactor deficiency and isolated sulfite oxidase deficiency. This test is much preferred over the use of sulfite dipstick tests that are only of utility if urine is fresh because sulfite is rapidly oxidized to sulfate at room temperature. Furthermore, the dipstick test lacks specificity and has been reported to give both false positive and negative results. The S-Sulfocysteine assay should be considered in the work up of individuals with neonatal seizures. A positive result with this assay should be confirmed by enzyme measurements in cultured fibroblasts and/or DNA testing.