SUOX and hyperinsulinemic hypoglycemia, familial, 4: One reason for the toxic potential of (bi)sulfite is a deficiency of sulfite oxidase, the molybdenum-containing enzyme that oxidizes sulfite to sulfate (SO42−), and it is noteworthy that in cases of sulfite oxidase deficiency, the concentration of sulfite in plasma is abnormal (> 1 mM) (Acosta et al. 1989; Johnson et al. 1980).