Physiological Role
Metabolism of Sulfur

I.  Physiological Role

  1. Component of sulfur amino acids – methionine, cystine, cysteine, homocyteine, cystathione
  2. Oxidation-reduction reactions
    1. The disulfide bond is easily converted to sulfhydryl groups and vice versa
      Cystine       2 Cysteine
    2. Biologically active compounds activity is dependent upon oxidation or reduction of SH groups:
      1. Insulin
      2. Glutathione (oxidation-reduction reactions; transport of amino acids)
      3. Coenzyme A (transport of acyl groups)
      4. Lipoic acid (one of coenzymes participating oxidation of a-ketoglutarate to succynyl CoA)
    3. Water soluble vitamins thiamin and biotin contain sulfur
    4. Component of the structural protein collagen
    5. Other important sulfur-containing materials
      1. Mucoitin sulfates (protect GI walls against digestive enzymes)
      2. Heparin (blood anti-coagulant in lung and artery walls)
      3. Fibrinogen (fibrin is framework for blood clot)
      4. Hemoglobin (transport of O2 and CO2 in blood)
      5. The cytochromes (electron transport, oxidative phosphorylation, detoxification or xenobiotics)
      6. Sulfates detoxify by forming esters

II.  Metabolism of Sulfur

  1. Ruminants
    1. Rumen microorganisms use inorganic sulfur to synthesize sulfur amino acids and vitamins
      1. Sulfate is reduced to sulfite by rumen bacteria
      2. Bacteria incorporate reduced sulfur into organic compounds
        1. Much of consumed S is converted to cysteine and methionine
        2. Organic S compounds are absorbed when bacteria are digested
    2. Large quantities of sulfide are also absorbed from the rumen
    3. Endogenous S is recycled via saliva to the rumen
    4. Sulfur from metabolism of S containing compounds is also excreted in the urine
  2. Nonruminants
    1. Only small amounts of inorganic S are usually present in diets of nonruminants
      1. Sulfur is largely ingested and absorbed as organic compounds
      2. Methionine, biotin, and thiamin are the only sulfur compounds required in the diet
      3. All other S compounds can be synthesized from products of methionine catabolism
    2. Absorption of sulfur
      1. Both inorganic and organic forms of S are absorbed by active transport from the small intestine
      2. Absorption of sulfate is usually 90% or greater
    3. Excretion of sulfur
      1. Inorganic sulfate (SO4)
      2. Neutral S (cystine, taurine, thiosulfates)
      3. Esterifed S (complex detoxification products)
  3. Interactions of sulfur with other elements
    1. Orbital structures are similar for S and Se
      1. Added dietary sulfate increases requirement for Se
      2. Se can replace S in methionine and cysteine
      3. Sulfate reduces toxic effects of excess Se
      4. S-Cu-Mo interaction will be discussed later
  4. Recommended allowance for sulfur:
    1. Ruminants: 0.1% - 0.2% in dietary DM
    2. Nonruminants: 0.3% - 0.8% sulfur amino acids in dietary DM
  5. Sources of sulfur
  6. Sulfur deficiency
    1. Ruminants
      1. Inadequate S nutrition of microbes
        1. Depressed diet digestibility
        2. Depressed microbial protein synthesis
        3. Suboptimal utilization of nonprotein nitrogen
        4. Reduced numbers of rumen microbes
      2. Effects on host animal
        1. Negative nitrogen balance
        2. Excessive tearing and salivation
        3. Dullness, emaciation, weakness
      1. Deficiencies of organic molecules that contain sulfur produce symptoms characteristic of the specific molecule that is deficient
  7. Sulfur excess
    1. Ruminants (J Dairy Sci. 67:2179, 1984)
      1. 0.4% S as sodium sulfate is believed to be maximum tolerable dietary S
      2. Causes of S toxicity in ruminants
        1. Use of gypsum to limit feed intake
        2. Use of ammonium sulfate as a major source of nonprotein nitrogen or to decrease cation-anion difference
      3. Clinical signs  of excess S
        1. Reduction in feed intake, ruminal digestion, rumen motility, and rumination
        2. Twitching of jaw, eyelid, and ear muscles
        3. Evidence of pain
        4. Staggery gait
        5. Severe diarrhea
        6. Advanced stages of S toxicity
          – Inability to rise
          – Grunting and fast, labored breathing
          – Coma and death
        7. Causes of death
          – Acute mortality: Action of hydrogen sulfide on respiratory and central nervous system
          – Later mortality: Dehydration from diarrhea
    2. Nonruminants
      1. May be less tolerant of sulfide than are ruminants.  Maximum tolerable level may be 0.2% - 0.3% dietary inorganic S
      2. Animals over liquid manure pits may be exposed to sulfur gases
      3. Effects on swine of inhaling air containing different amounts of hydrogen sulfide:
        1. 0 – 9 ppm.........None
        2. 5 – 10 ppm.......Temporary eye irritation, nasal discharge, shallow breathing
        3. 28 ppm.............Loss of sense of smell
        4. 47 ppm.............Discomfort
        5. 180 ppm...........Severe toxicity
        6. 447 ppm...........Death

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