Changes

'''Methionine is a neutral amino acid''', while '''cysteine is basic'''; both methionine and cysteine are '''gluconeogenic'''. Cysteine readily reacts with itself and other thiols (i.e. sulphur containing compounds) and cystine is formed from a disulphide bond between two cysteine molecules. Most of the plasma cysteine is actually found as cystine.

'''Methionine is a neutral amino acid''', while '''cysteine is basic'''; both methionine and cysteine are '''gluconeogenic'''. Cysteine readily reacts with itself and other thiols (i.e. sulphur containing compounds) and cystine is formed from a disulphide bond between two cysteine molecules. Most of the plasma cysteine is actually found as cystine.

Dietary methionine is absorbed by a neutral amino acid transporter in the small intestine (particularly the [[Jejunum - Anatomy & Physiology|jejunum]]) and plasma methionine is actively reabsorbed in the proximal tubule of the [[Renal Anatomy - Anatomy & Physiology|kidney]]; dietary cysteine and cystine are absorbed in the small intestine and actively reabsorbed in the proximal tubule of the kidney via a dibasic amino acid transporter.

Dietary methionine is absorbed by a neutral amino acid transporter in the [[Small Intestine Overview - Anatomy & Physiology|small intestine]] (particularly the [[Jejunum - Anatomy & Physiology|jejunum]]) and plasma methionine is actively reabsorbed in the [[Nephron Microscopic Anatomy#Proximal Tubule|proximal tubule]] of the [[Renal Anatomy - Anatomy & Physiology|kidney]]; dietary cysteine and cystine are absorbed in the small intestine and actively reabsorbed in the proximal tubule of the kidney via a dibasic amino acid transporter.

==Why are they Important?==

==Why are they Important?==

Both methionine and cysteine are incorporated into structural protein and are '''required for normal growth'''. The sulphur side chains help stabilize secondary and tertiary protein structures. Methionine is part of the coenzyme S-adenosyl methionine, which influences and regulates the activity of a number of enzymatic and cellular replication processes.

Both methionine and cysteine are incorporated into structural protein and are '''required for normal growth'''. The sulphur side chains help stabilize secondary and tertiary protein structures. Methionine is part of the coenzyme S-adenosyl methionine, which influences and regulates the activity of a number of enzymatic and cellular replication processes.

Inherited defects in the transporter for dibasic amino acids can result in poor absorption of cyst(e)ine (as well as the other dibasic amino acids [[Lysine - Nutrition|lysine]], ornithine, and [[Arginine - Nutrition|arginine]]) from the intestinal mucosa and poor reabsorption in the renal tubule<ref name="Hoppe">Hoppe A, et al. Urinary excretion of amino acids in normal and cystinuric dogs. Br Vet J 1993;149:253-68.</ref>. Unlike lysine, ornithine and arginine, cystine is not soluble in urine and readily forms crystals and stones. Cystinuria and related dysuria and urinary obstructions due to cystine urolithiasis have been described in Newfoundlands, English bulldogs, and Dachshunds<ref name="Brons">Brons AK, et al. SLC3A1 and SLC7A9 mutations in autosomal recessive or dominant canine cystinuria: A new classification system. JVIM 2013;27:1400-1408.</ref>. Increased intake of DL-methionine (either in the diet or as a supplement) has been used as a therapeutic treatment for sterile struvite crystalluria and urolithiasis<ref name="Lemann">Lemann J and Relman AS. The relation of sulfur metabolism to acid-base balance and electrolyte excretion: the effects of DL-methionine in normal man. J Clin Invest 1959;38:2215-2223.</ref><ref name="Mishina">Mishina M et al. Medical dissolution of struvite nephrolithiasis using amino acid preparations in dogs. JVIM 2000;62:889-892.</ref>. The oxidation of dietary sulphur increases urinary excretion of ammonium (NH₄⁺) resulting in a more acidic urinary pH.

Inherited defects in the transporter for dibasic amino acids can result in poor absorption of cyst(e)ine (as well as the other dibasic amino acids [[Lysine - Nutrition|lysine]], ornithine, and [[Arginine - Nutrition|arginine]]) from the intestinal mucosa and poor reabsorption in the renal tubule<ref name="Hoppe">Hoppe A, et al. Urinary excretion of amino acids in normal and cystinuric dogs. Br Vet J 1993;149:253-68.</ref>. Unlike lysine, ornithine and arginine, cystine is not soluble in urine and readily forms [[Urolithiasis|crystals and stones]]. Cystinuria and related dysuria and urinary obstructions due to cystine urolithiasis have been described in Newfoundlands, English bulldogs, and Dachshunds<ref name="Brons">Brons AK, et al. SLC3A1 and SLC7A9 mutations in autosomal recessive or dominant canine cystinuria: A new classification system. JVIM 2013;27:1400-1408.</ref>. Increased intake of DL-methionine (either in the diet or as a supplement) has been used as a therapeutic treatment for sterile struvite crystalluria and urolithiasis<ref name="Lemann">Lemann J and Relman AS. The relation of sulfur metabolism to acid-base balance and electrolyte excretion: the effects of DL-methionine in normal man. J Clin Invest 1959;38:2215-2223.</ref><ref name="Mishina">Mishina M et al. Medical dissolution of struvite nephrolithiasis using amino acid preparations in dogs. JVIM 2000;62:889-892.</ref>. The oxidation of dietary sulphur increases urinary excretion of ammonium (NH₄⁺) resulting in a more acidic urinary pH.

==Roles in the Body==

==Roles in the Body==

Methionine is part of the coenzyme s-adenosylmethionine, which through its ability to transfer to and methylate other substrates, is able to modify the activities of a range of different metabolic processes including nucleic acids, proteins, lipds and secondary metabolites); it is also a constituent of protein, and a precursor of cysteine<ref name="Stipanuk">Stipanuk MH and Watford M. Amino acid metabolism. In Biochemical and physiologic aspects of human nutrition. 2000 Philidelphia, PA: WB Saunders Company p. 265-270.</ref>.  

Methionine is part of the coenzyme s-adenosylmethionine, which through its ability to transfer to and methylate other substrates, is able to modify the activities of a range of different metabolic processes including nucleic acids, proteins, lipds and secondary metabolites); it is also a constituent of protein, and a precursor of cysteine<ref name="Stipanuk">Stipanuk MH and Watford M. Amino acid metabolism. In Biochemical and physiologic aspects of human nutrition. 2000 Philidelphia, PA: WB Saunders Company p. 265-270.</ref>.  

Cysteine readily forms sulphide bonds with other thiol groups stabilizing secondary and tertiary structure in proteins such as hair, glutathione, and insulin; and acts as a sulphur donor to choline, an essential vitamin-like nutrient<ref name="NRC">National Research Council (NRC). Protein and Amino Acids. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p. 125-126.</ref>. Cysteine is a precursor to [[Taurine - Nutrition|taurine]] in dogs, however cats have low activity of hepatic cysteine dioxygenase and cysteine sulphinate decarboxylase activity (two key enzymes in the conversion of cysteine to taurine) and require a preformed source of taurine in the diet<ref name="De la Rosa">De la Rosa J, et al. Metabolism of cysteine and cyteinesulfinate in rat and cat hepatocytes. J Nutr 1987;117:549-558.</ref>. Cysteine is also a precursor of felinine, a urine pheromone produced by cats that gives cat urine its distinctive aroma<ref name="Hendriks">Hendriks WH, et al. Importance of sulfate, cysteine and methionine as precursors to felinine synthesis by domestic cats (Felis catus). Comp Biochem Physiol C Toxicol Pharmacol 2001;129:211–216.</ref>.

Cysteine readily forms sulphide bonds with other thiol groups stabilizing secondary and tertiary structure in proteins such as hair, glutathione, and [[insulin]]; and acts as a sulphur donor to choline, an essential vitamin-like nutrient<ref name="NRC">National Research Council (NRC). Protein and Amino Acids. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p. 125-126.</ref>. Cysteine is a precursor to [[Taurine - Nutrition|taurine]] in dogs, however cats have low activity of hepatic cysteine dioxygenase and cysteine sulphinate decarboxylase activity (two key enzymes in the conversion of cysteine to taurine) and require a preformed source of taurine in the diet<ref name="De la Rosa">De la Rosa J, et al. Metabolism of cysteine and cyteinesulfinate in rat and cat hepatocytes. J Nutr 1987;117:549-558.</ref>. Cysteine is also a precursor of felinine, a urine pheromone produced by cats that gives cat urine its distinctive aroma<ref name="Hendriks">Hendriks WH, et al. Importance of sulfate, cysteine and methionine as precursors to felinine synthesis by domestic cats (Felis catus). Comp Biochem Physiol C Toxicol Pharmacol 2001;129:211–216.</ref>.

==Consequences of Methionine and Cysteine Deficiency==

==Consequences of Methionine and Cysteine Deficiency==

[[Category:Amino Acids]]

[[Category:Amino Acids]]

[[Category:To Do - Nutrition]]

[[Category:To Do - Nutrition]]

[[Category:To Do - Nutrition GGP]]

[[Category:To Do - Nutrition preMars]]