Contributor: Gordon K. Klintworth
Sulfocysteinuria (sulfite oxidase deficiency) is an extremely rare inherited autosomal recessive disorder in which sulfite oxidase is deficient due to a mutation in the SUOX gene. It is characterized clinically by severe neurological defects, which include profound mental deficiency, myoclonus, and seizures. Bilateral dislocated lenses are a prominent feature in ~50% of cases and probably occurs because disulfide links form important bonds in fibrillin, a major constituent of zonules. A similar mechanism may contribute to the dislocated lenses in homocystinuria. Other ocular abnormalities (hypoplasia of ciliary body [hypoplasia - ciliary body], diminished ganglion cells and thinning of the nerve fiber layer in the retina, and absence of  myelin in the optic nerve) have also been noted in sulfocysteinuria. There is an increased urinary excretion of sulfite ions, thiosulfate ions, and the amino acid S-sulfo-L-cysteine, but sulfate ions in the urine are markedly diminished or absent. S-sulfo-L-cysteine may also be elevated in the plasma. Affected patients have markedly high levels of sulfite ions, which destroy disulfide bonds and react with free sulfhydryl groups. Affected individuals frequently inherit a combined deficiency of both sulfite oxidase and xanthine oxidase (molybdenum cofactor deficiency) because of an inherited deficiciency of molybdenum cofactor that both enzymes require.