Sulfur occurs principally in the body as a constituent of the cysteine and methionine. As a constituent of these amino acids it plays an important role in the structural conformation of almost all proteins and enzymes found in the body.
Sulfur is also important in the structure of antibodies, and the sulfide group of cysteine is important in the activity of various enzymes. The disulfide linkage, which is formed between two cysteine residues, tends to bind up adjacent parallel peptide chains, thereby adding to the structural rigidity of many proteins.
Sulfur is an important constituent of thiamine and biotin, and is found in insulin, keratin, and various glycoproteins. It is important in the molecule chondroitin sulfate in bone and cartilage.
Method of Action
Sulfur in the body exist primarily as a constituent of cysteine and methionine, although it also exists in smaller amounts in various inorganic sulfate compounds.
Sulfate, which contains sulfur, is an important constituent of the compound serotonin; serotonin acts as a vasoconstrictor and aids in blood coagulation.
Disulfide bridges, or sulfur to sulfur bonding of two cysteine residues, function to link together the various peptide constituents of all immunoglobulin molecules. Disulfide bridges also aid in the maintenance of structural rigidity in keratin protein. Keratin is composed of many peptide chains, which are linked together by hydrogen and disulfide bonds. Skin is made of a "softer" keratin which contains less disulfide bonds and, therefore, less structural rigidity. "Hard" keratin, as is found in hair and nails, has a greater amount of structural rigidity.
Disulfide bonds are essential in maintaining the three dimensional structures of proteins and enzymes. Enzymatic activity, in many enzymes, depends upon the protein structure. For example, pancreatic ribonuclease has four disulfide bonds within its protein structure and does not function efficiently in the absence or malformation of these bonds.
Sulfur is a constituent of the molecule coenzyme A (CoA), which is involved in a wide variety of metabolic reactions. CoA plays a role in glycolysis, metabolism of fatty acids, degradation of various amino acids, and the formation of the neurotransmitter acetylcholine, as well as numerous other metabolic processes.
Properties & Uses
Sulfur-containing drugs have been used in a wide variety of clinical treatments. Among these are the sulfonylureas, which enhance glucose utilization in diabetic patients; sulfonamides, used in combatting infection; salicylazosulfapyradine, an anti-inflammatory agent; sulfisoxazole, an antibiotic; sulfinpyrazone, which helps treat chronic gout and recurrent heart attacks; and sulfasalazine, which is used in the treatment of ulcers.
Consequence of Deficiency
Cysteine renal calculi arise from a hereditary defect in renal tubular resorption of cysteine, which results in cystinuria (an abnormally high urinary excretion of cysteine) and production of cysteine crystal kidney stones.
It should be noted that adequate protein intake almost always assures adequate organic sulfur intake.
Ingestion of organically-complexed sulfur (e.g., in amino acids) in excess has not been shown to be toxic. High sulfate levels have been observed in infants fed exclusively on formulae with abnormally high cysteine and methionine contents.
Recommended Dietary Allowance
There has been no RDA established for sulfur. It is assumed an individual takes in sufficient amounts of sulfur when the protein intake is adequate.
For over thirty years, Recommended Daily Amounts has existed in the United Kingdom. It has been used to measure the adequacy of an individual's diet. However, in 1991 the Committee on Medical Aspects of Food Policy (COMA) gave forth a whole new set of figures upon the request of the Department of Health's Chief Medical Officer. Reference Nutrient Intake (RNI) is one of these sets collectively known as "Dietary Reference Values." RNI is an amount of a nutrient that is enough for almost every individuals, even someone who has high needs for the nutrient. This level of intake is, therefore, considerably higher than what most people would need. If individuals are consuming the RNI of a nutrient they are most unlikely to be deficient in that nutrient.
The major food sources are proteins containing methionine and cysteine. The body synthesizes cysteine from its precursor methionine.
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