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==What are Phenylalanine and Tyrosine?==

==What are Phenylalanine and Tyrosine?==

Phenylalanine and tyrosine are '''aromatic amino acids each containing a benzene ring side chain'''. Phenylalanine is converted to tyrosine in both dogs and cats and '''only phenylalanine is considered an essential amino acid'''. Tyrosine production accounts for half of the total phenylalanine requirement in the diet<ref name ="Milner">Milner JA. Phenylalanine and tyrosine requirement in immature beagle dogs. J Nutr 1984;114:2212-2216.</ref>, and both amino acids are considered together when determining the daily requirement. Phenylalanine is converted directly to tyrosine via phenylalanine hydroxylase in the [[Liver - Anatomy & Physiology|liver]]; this is a non-reversible step in phenylalanine degradation. The addition of dietary tyrosine can “spare” phenylalanine reducing the phenylalanine requirement. Phenylalanine and tyrosine are '''neutral amino acids and are both gluconeogenic and ketogenic'''. They are absorbed by a neutral amino acid transporter in the [[Small Intestine Overview - Anatomy & Physiology|small intestine]] (particularly the [[Jejunum - Anatomy & Physiology|jejunum]]) and are actively reabsorbed in the [[Nephron Microscopic Anatomy#Proximal Tubule|proximal tubule]] of the kidney.

Phenylalanine and tyrosine are '''[[Nutrition Glossary#Aromatic Amino Acids|aromatic amino acids]] each containing a benzene ring side chain'''. Phenylalanine is converted to tyrosine in both dogs and cats and '''only phenylalanine is considered an [[Amino Acids Overview - Nutrition|essential amino acid]]'''. Tyrosine production accounts for half of the total phenylalanine requirement in the diet<ref name ="Milner">Milner JA. Phenylalanine and tyrosine requirement in immature beagle dogs. J Nutr 1984;114:2212-2216.</ref>, and both amino acids are considered together when determining the daily requirement. Phenylalanine is converted directly to tyrosine via phenylalanine hydroxylase in the [[Liver - Anatomy & Physiology|liver]]; this is a non-reversible step in phenylalanine degradation. The addition of dietary tyrosine can “spare” phenylalanine reducing the phenylalanine requirement. Phenylalanine and tyrosine are '''neutral amino acids and are both [[Amino Acids Overview - Nutrition|gluconeogenic and ketogenic]]'''. They are absorbed by a neutral amino acid transporter in the [[Small Intestine Overview - Anatomy & Physiology|small intestine]] (particularly the [[Jejunum - Anatomy & Physiology|jejunum]]) and are actively reabsorbed in the [[Nephron Microscopic Anatomy#Proximal Tubule|proximal tubule]] of the kidney.

==Why are they Important?==

==Why are they Important?==

==Roles in the Body==

==Roles in the Body==

Tyrosine is hydroxylated to 3,4-dihydroxyphenylalanine (DOPA) by tyrosine hydroxylase within different tissues. Depending on where DOPA is produced it can be further converted to dopamine and norepinephrine (e.g., brain and nervous tissue) or melanin (i.e., melanocytes); iodinated tyrosine residues on thyroglobulin also help form triiodothyronine (T3) and thyroxine (T4) molecules<ref>Stipanuk MH and Watford M. Amino acid metabolism. In Biohemical and physiologic aspects of human nutrition. 2000 Philidelphia, PA: WB Saunders Company p. 270-274.</ref>.

Tyrosine is hydroxylated to 3,4-dihydroxyphenylalanine (DOPA) by tyrosine hydroxylase within different tissues. Depending on where DOPA is produced it can be further converted to dopamine and norepinephrine (e.g. brain and nervous tissue) or melanin (i.e. melanocytes); iodinated tyrosine residues on thyroglobulin also help form triiodothyronine (T3) and thyroxine (T4) molecules<ref>Stipanuk MH and Watford M. Amino acid metabolism. In Biohemical and physiologic aspects of human nutrition. 2000 Philidelphia, PA: WB Saunders Company p. 270-274.</ref>.

Tyrosine is a precursor to melanin in hair. Twice the amount of phenylalanine and tyrosine are required to produce and maintain a normal black hair coat colour than are required for growth in both dogs and cats<ref name="Biourge">Biourge V and Sergheraert R. Hair pigmentation can be affected by diet in dogs.  Proc Comp Nutr Soc 2002; 4:103-104.</ref><ref name="Yu">Yu SC, et al. Effect of low levels of dietary tyrosine on the hair colour of cats. J Sm Anim Pra 2001;42:176-180.</ref><ref name="Anderson">Anderson PA, et al. Histidine, phenylalanine-tyrosine and tryptophan requirements for growth of the young kitten. J Anim Sci 1980;50:266-271.</ref>.

Tyrosine is a precursor to melanin in [[Hair - Anatomy & Physiology|hair]]. Twice the amount of phenylalanine and tyrosine are required to produce and maintain a normal black hair coat colour than are required for growth in both dogs and cats<ref name="Biourge">Biourge V and Sergheraert R. Hair pigmentation can be affected by diet in dogs.  Proc Comp Nutr Soc 2002; 4:103-104.</ref><ref name="Yu">Yu SC, et al. Effect of low levels of dietary tyrosine on the hair colour of cats. J Sm Anim Pra 2001;42:176-180.</ref><ref name="Anderson">Anderson PA, et al. Histidine, phenylalanine-tyrosine and tryptophan requirements for growth of the young kitten. J Anim Sci 1980;50:266-271.</ref>.

The presence of phenylalanine-containing peptides in the intestinal lumen is a trigger for release of cholecystokinin (CCK)<ref>Koop I and Buchan AM. Cholecystokinin release from isolated canine epithelial cells in short-term culture. Gastroenter 1992;102:28-34.</ref><ref>Backus RC, et al. The potency of dietary amino acids in elevating plasma cholecystokinin immunoreactivity in cats is related to amino acid hydrophobicity. Regul Pept 1997;72:31-40.</ref>.

The presence of phenylalanine-containing peptides in the intestinal lumen is a trigger for release of cholecystokinin (CCK)<ref>Koop I and Buchan AM. Cholecystokinin release from isolated canine epithelial cells in short-term culture. Gastroenter 1992;102:28-34.</ref><ref>Backus RC, et al. The potency of dietary amino acids in elevating plasma cholecystokinin immunoreactivity in cats is related to amino acid hydrophobicity. Regul Pept 1997;72:31-40.</ref>.

==Dietary Sources==

==Dietary Sources==

Sufficient phenylalanine is found in plant and animal protein sources, such as muscle meat, eggs, dairy protein (e.g., casein), cereal grains, and pulses (i.e., legumes).

Sufficient phenylalanine is found in plant and animal protein sources, such as muscle meat, eggs, dairy protein (e.g. casein), cereal grains, and pulses (i.e. legumes).

==Diagnosing Phenylalanine and Tyrosine Deficiency==

==Diagnosing Phenylalanine and Tyrosine Deficiency==

==References==

==References==

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[[Category:To Do - Nutrition preMars]]

[[Category:Amino Acids]]