In mammals, the expression levels of sulfite oxidase is high in the liver, kidney, and heart, and very low in spleen, brain, skeletal muscle, and blood.

The molybdenum centre has a square pyramidal geometry and is distinguished from the xanthine oxidase family by the orientation of the oxo group facing downwards rather than up.

The active site of sulfite oxidase contains the molybdopterin cofactor and supports molybdenum in its highest oxidation state, +6 (MoVI).

In the enzyme's oxidized state, molybdenum is coordinated by a cysteine thiolate, the dithiolene group of molybdopterin, and two terminal oxygen atoms (oxos).

This rare but fatal disease causes neurological disorders, mental retardation, physical deformities, the degradation of the brain, and death.

Reasons for the lack of functional sulfite oxidase include a genetic defect that leads to the absence of a molybdopterin cofactor and point mutations in the enzyme.