Sulphite oxidase (gene name: SUOX) is a molybdenum – containing enzyme that consists of 466 amino acids. It belongs to the xanthine dehydrogenase family and is found in almost all organisms. Its center contains molybdenum, a trace element essential to life .

The metal occurs there in its bioavailable form as molybdate anion. The sulfite oxidase uses it as a cofactor (molybdate-molybdopterin compound). The enzyme converts the sulphur-containing amino acids methionine , cysteine , etc. taken in with food into harmless sulfur salts (sulphates), which are then excreted in the urine. In mammals, the sulfur-degrading biocatalyst is found primarily in the liver and kidneys . The enzyme sulfite oxidase ensures that the blood oxygen combines with the essential amino acids and other sulfur substances.

The electrons released in the process are used via the electron transport chain to obtain ATP ( adenosine triphosphate ). The enzyme catalyzes 10 times the amount of sulfites contained in one liter of alcohol every day .

Function, effect & tasks

Everyone consumes sulfur-containing proteins and food additives on a daily basis. The latter are contained, for example, in pickled vegetables, grapefruit juice, etc. and are intended to protect the food from microbial infestation and discoloration. They form bouquet substances in wine. 

Just by breaking down cysteine , the body produces 1680 mg of toxic sulfite every day, which must be converted immediately by sulfite oxidase so that organs and tissue are not damaged. The enzyme works together with other biocatalysts. Sulfites are toxic and can destroy vital substances in the body and inhibit necessary metabolic processes even in the smallest amounts. In order to be able to carry out the important detoxification of the cells , the sulfite oxidase needs the trace element molybdenum.

A lack of this metal can lead to serious consequences. Too high a mercury content in the body can also inhibit the functionality of sulfite oxidase.

Formation, Occurrence, Properties & Optimal Values

Sulphite oxidase is primarily made in the mitochondria , the “energy centers” of cells. In rats, for example, 80 percent of it is found in the liver cell mitochondria. In addition, it is still strongly represented in the cells of the kidneys .

The molybdenum oxide required for the activity of sulfite oxidase is located in the active center of the enzyme. As scientists recently discovered, it can be replaced with molybdenum trioxide nanoparticles in patients with a molybdenum deficiency. They have a similar catalytic effect in the body as the natural enzyme. In this way, previously fatal diseases such as sulfocysteinuria could be treated.

Diseases & Disorders

A deficiency in sulfite oxidase can cause asthmatic and even anaphylactic reactions in otherwise healthy people by the parasympathetic nervous system affecting the mast cells responsible for causing allergies .

In addition, due to a low sulfite oxidase level, severe tiredness , headaches and low blood sugar levels can occur. However, the genetically determined lack of the vital enzyme has even worse consequences. The newborn is born with physical deformities and mental disabilities. This so-called sulfite oxidase deficiency or sulfocysteinuria occurs as a molybdenum cofactor (MoCo) deficiency disease in an estimated one in 100,000 to 500,000 births. Infants suffering from an isolated sulfite oxidase defect show similar symptoms: severe encephalopathy, barely controllable seizures , spasticity , microcephaly, muscle relaxation and progressive cerebral atrophy .

Since the autosomal recessive inherited enzyme deficiency disease cannot currently be treated effectively, the young patients usually die in childhood: the sulfurous compounds that are not broken down poison neurons and myelin sheaths of the central nervous system and accumulate in the cell tissue. Even after birth , there are problems with eating and vomiting of the stomach contents. The infants are born with a deformed skull (protruding forehead, deep-set eyes, excessively long eyelids, thick lips, small nose). During the first few months of life, the lens in the eye shifts .

About 75 percent of the cases of sulfocysteinuria described so far are due to a MoCo deficiency: all three enzymes involved in sulfur breakdown in the body, sulfite oxidase, xanthine oxidase and aldehyde oxidase, show a greatly reduced activity. A mutation is to blame for the isolated sulfite oxidase deficiencyin the SUOX gene (chromosome 12). It appears in three variants: type A (mutation in the MOCS1 gene), type B (MOCS2 gene) and type C (MOCS3 gene). The type A mutation is the most common. In this case, the formation of the precursor molecule cPMP is inhibited. However, the substance can now be produced and administered in the laboratory. In order to improve the survival of the child patient, the deficiency disease should be diagnosed as soon as possible and treated with daily intravenous administrations of molybdate.

This can at least contain further damage. The child is given antispasmodic medication to stop the seizures. In addition, it must be on a low-protein diet . Alternatively, the MoCo precursor Z can also be administered. It reduces seizures and prevents further brain damage. Medicine has high hopes for the treatment of the hitherto incurable disease with molybdenum trioxide nanoparticles, which take on the role of sulfite oxidase in the body. To find out whether the unborn child has a sulfite oxidase deficiency, the pregnant woman can have her S-sulfocysteine ​​level examined in the amniotic fluid .