Endogenous sulfite is generated as a consequence of the body's normal processing of sulfur-containing amino acids. Sulfites occur as a consequence of fermentation and also occur naturally in a number of foods and beverages. As food additives, sulfiting agents were first used in 1664 and have been approved in the United States since the 1800s. Sulfite is an allergen, a neurotoxin, and a metabotoxin.  An allergen is a compound that causes allergic reactions such as wheezing, rash, or rhinitis. A neurotoxin is a substance that causes damage to nerves or brain tissues. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels. As an allergen, sulfite is known to induce asthmatic reactions. Sulfite sensitivity occurs most often in asthmatic adults (predominantly women), but it is also occasionally reported in preschool children. Adverse reactions to sulfites in nonasthmatics are extremely rare. Asthmatics who are steroid-dependent or who have a higher degree of airway hyperreactivity may be at greater risk of experiencing a reaction to sulfite-containing foods. Sulfite sensitivity reactions vary widely, ranging from no reaction to severe. The majority of reactions are mild. These manifestations may include dermatologic, respiratory, or gastrointestinal signs and symptoms. The precise mechanisms of the sensitivity responses have not been completely elucidated: inhalation of sulfur dioxide (SO2) generated in the stomach following ingestion of sulfite-containing foods or beverages, a deficiency in a mitochondrial enzyme, and an IgE-mediated immune response have all been implicated. Exogenously supplied sulfite is detoxified by the enzyme sulfite oxidase. Sulfite oxidase (EC 1.8.3.1) is 1 of 3 enzymes in humans that require molybdenum as a cofactor. Under certain circumstances, chronically high levels of sulfite can lead to serious neurotoxicity. Sulfite oxidase deficiency (also called molybdenum cofactor deficiency) is a rare autosomal inherited disease that is typified by high concentrations of sulfite in the blood and urine. It is characterized by severe neurological symptoms such as untreatable seizures, attenuated growth of the brain, and mental retardation. It results from defects in the enzyme sulfite oxidase, which is responsible for the oxidation of sulfite to sulfate. This sulfite to sulfate reaction is the final step in the degradation of sulfur-containing metabolites (including the amino acids cysteine and methionine). The term "isolated sulfite oxidase deficiency" is used to define the deficiency caused by mutations in the sulfite oxidase gene. This differentiates it from another version of sulfite oxidase deficiency that is due to defects in the molybdenum cofactor biosynthetic pathway (with mutations in the MOCS1 or MOCS2 genes). Isolated sulfite oxidase deficiency is a rare but devastating neurologic disease that usually presents in early infancy with seizures and alterations in muscle tone (PMID: 16234925, 16140720, 8586770). Sulfite oxidase deficiency (as caused by MOCS1 or MOCS2) may be treated with cPMP, a precursor of the molybdenum cofactor (PMID: 20385644).  The mechanism behind sulfite neurotoxicity appears to be related to its ability to bind and inhibit glutamate dehydrogenase (GDH). Inhibition of GDH leads to a decrease in alpha-ketoglutarate and a diminished flux through the tricarboxylic acid cycle. This is accompanied by a decrease in NADH through the mitochondrial electron transport chain, which leads to a decrease in mitochondrial membrane potential and in ATP synthesis. Since glutamate is a major metabolite in the brain, inhibition of GDH by sulfite appears to contribute to neural damage characteristic of sulfite oxidase deficiency in human infants (PMID: 15273247).