Phenylalanine and Tyrosine - Nutrition
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, and both amino acids are considered together when determining the daily requirement. Phenylalanine is converted directly to tyrosine via phenylalanine hydroxylase in the 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 (particularly the jejunum) and are actively reabsorbed in the proximal tubule of the kidney.
Why are they Important?
Phenylalanine and tyrosine are incorporated into structural protein and the presence of the hydrophobic aromatic side-chain will induce a fold in the protein structure. In addition to being required for normal growth, both phenylalanine and tyrosine are further incorporated into key regulatory hormones and compounds. Phosphorylation of tyrosine residues by tyrosine kinase plays a role in cellular replication and signalling and abnormal tyrosine kinase activity has been associated with loss of cellular regulation and development of a number of cancers in dogs.
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. 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. The presence of phenylalanine-containing peptides in the intestinal lumen is a trigger for release of cholecystokinin (CCK).
Consequences of Phenylalanine and Tyrosine Deficiency
Puppies fed a phenylalanine deficient diet experience decreased food intake and weight loss. Adults dogs fed inadequate phenylalanine and tyrosine levels in the diet develop a reddening of the haircoat.
Feeding a phenylalanine deficient diet to kittens results in weight loss, while no change in weight gain is seen when tyrosine is deficient but adequate phenylalanine levels are present. Kittens fed adequate phenylalanine for growth, but suboptimal amounts for melanin production develop reddening of the haircoat, which can progress to ataxia, vocalizing, ptylism, hyperactivity, and abnormal tail posture (tail held bent forward) if continued for more than 6 months.
There are no studies on either acute or chronic toxicity related to feeding high doses of phenylalanine or tyrosine to dogs and no reports of safe upper limits. Kittens fed purified diets containing phenylalanine and tyrosine 4x the requirement for maintenance of black haircoat showed no adverse effects, but when tyrosine level was increased to 8x the requirement for normal black coat colour a decrease in food intake was seen.
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
Diagnosis of phenylalanine and tyrosine deficiency is based on fasted plasma amino acids and coat colour changes.
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Date reviewed: 18 May 2015
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