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Sulfur (chemical symbol S) is one of the essential trace elements. It has an important biological role as part of sulfur-containing amino acids and other important biochemical species such as glutathione, hydrogen sulfide, and coenzyme A.
Sulfur has been the preferred spelling of the International Union of Pure and Applied Chemistry (IUPAC) since 1990, and is also preferred by many scientific journals and Radiopaedia 2. The alternative spelling sulphur may still be found in common use in the UK and Commonwealth, especially by laypeople. By extension all sulfur-derived compounds are also spelled using the 'f' form e.g. sulfuric acid, disulfite etc. "Thiol" refers to a specific chemical side group containing sulfur.
Sulfur has the atomic number 16 with an atomic weight of 32.064 g/mol. It is a non-metallic, yellowish, odorless, flavorless element. By contradistinction its compounds are well-known for their pungent - often offensive - aromas.
There are four stable isotopes of sulfur, which are S-32, S-33, S-34 and S-36. Sulfur-32 accounts for 95% of the element on earth, with sulfur-34 accounting for most of the rest. Twenty other unstable isotopes of sulfur are known to exist, most with short half lives (<1 hour) 7.
The daily dietary requirement of sulfur-containing amino-acids (SAAs) is recommended as ~13–15 mg/Kg 5. The majority of this is as the compound cystine, a disulfide oxidised derivative of cysteine.
Of the important sulfur-containing amino acids, methionine can only be obtained from the diet, whilst cysteine can be chemically derived from methionine via the transsulfuration pathway, as explained later. Sulfur in the diet is also sourced from taurine and cystine. Taurine has been a popular additive to energy drinks, although evidence for its efficacy is not clear-cut.
Several important food sources for sulfur are garlic, onions, and other vegetables, which are rich in sulfur-containing compounds, e.g. isothiocyanates, diallyl sulfide and allicin.
Absorption, transport and storage
Sulfur is generally stored in the cells as the oligopeptide glutathione (GSH). Cysteine and methionine are not stored by the body, and if there is an excess they pass into the urine, are oxidised to sulfate or metabolized to glutathione to be stored 6.
The amino acids methionine, cysteine, and homocysteine (and their derived forms cystine and homocystine) are all sulfur-containing amino acids (SAAs). Like other amino acids, the SAAs are important for the building up of polypeptides and proteins.
Methionine is metabolized to homocysteine, and via the transsulfuration pathway to cysteine. Cysteine is a precursor for the synthesis of glutathione (GSH), a key tripeptide intracellular redox agent neutralising free radicals and toxins, and acting as an enzymatic cofactor.
Coenzyme A (CoA) and taurine, are other physiologically important sulfur-containing moieties created from cysteine. Hydrogen sulfide (H2S) is created at several points along the transsulfuration pathway.
Methionine also donates methyl side groups for methylation e.g. gene regulation.
Toxicity and deficiency
As dietary sulfur is not consumed in the elemental form, excessive or deficient elementary sulfur is not generally an issue. However toxic intakes of both methionine or cysteine/cystine are well-recognized with potentially marked deleterious effects, e.g. on brain development 6.
Sulfur is used in nuclear medicine in the synthesis of sulfur-colloid compounds. Eight atoms of sulfur are arranged in a ring, with the IUPAC name of octathiocane, and complexed with a single technetium atom (formula: S8Tc) 8.
History and etymology
Although sulfur has been known since ancient times, the renowned French chemist Antoine Laurent Lavoisier (1743-1794) 4 was the first person to realize that sulfur was a chemical element in 1777. Unfortunately he met a rather premature end under the guillotine during the French revolution.
Sulfur and thiol are from the Latin and Ancient Greek words respectively for brimstone, the ancient term for sulfur. The Latin word 'sulfur' was identical, whilst 'thiol' derives from the Ancient Greek word θεῖον (theion) 3.