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Tetrahydrobiopterin deficiency (THBD, BH 4 D) is a rare metabolic disorder that increases the blood levels of phenylalanine.Phenylalanine is an amino acid obtained normally through the diet, but can be harmful if excess levels build up, causing intellectual disability and other serious health problems.
Phenylketonuria (PKU)-like symptoms, including more pronounced developmental defects, skin irritation, and vomiting, may appear when phenylalanine levels are near 20 mg/dL (1200 mol/L). [1] Hyperphenylalaninemia is a recessive hereditary metabolic disorder that is caused by the body's failure to convert phenylalanine to tyrosine as a result of ...
Type I tyrosinemia can be detected via blood tests for the presence of a fumarylacetoacetate metabolite, succinylacetone, which is considered a pathognomonic indicator for the disease. [ 6 ] Type II tyrosinemia can be detected via the presence of significantly elevated plasma tyrosine levels, and the diagnosis can be confirmed by detection of a ...
Patients received amino acid supplements lacking tyrosine and phenylalanine, most often by drinking a specially engineered formula, in order to acquire sufficient protein. It is recommended that tyrosine levels remain below 500 μmol/L. [5] Phenylalnine is the precursor to tyrosine. The ideology behind maintaining low tyrosine levels is two-fold.
The absence of BH 4 affects the metabolism of Phenylalanine. This is the reason that PKU and PTPS deficiency share some similar symptoms. However, since BH 4 is needed for much more than just the metabolism of Phenylalanine, there are other symptoms as well. [7]
Type II tyrosinemia is caused by a deficiency of the enzyme tyrosine aminotransferase (EC 2.6.1.5), encoded by the gene TAT.Tyrosine aminotransferase is the first in a series of five enzymes that converts tyrosine to smaller molecules, which are excreted by the kidneys or used in reactions that produce energy.
Phenylalanine is a large, neutral amino acid (LNAA). LNAAs compete for transport across the blood–brain barrier (BBB) via the large neutral amino acid transporter (LNAAT). If phenylalanine is in excess in the blood, it will saturate the transporter. Excessive levels of phenylalanine tend to decrease the levels of other LNAAs in the brain.
According to a review of dopa-responsive dystonias published in 2021, tyrosine hydroxylase deficiency may be hard to diagnose, with a median diagnostic delay of 4 years, [2] and misdiagnosis happens in a significant proportion of patients, with cerebral palsy being the most common erroneous diagnosis.