Abstract
Tetrahydrobiopterin (BH4) cofactor is essential for various enzyme activities, including phenylalanine-4-hydroxylase, tyrosine-3-hydroxylase, tryptophan-5-hydroxylase, nitric oxide synthase, and glyceryl-ether monooxygenase. Measuring of pterins in different biological fluids is the most common method for screening and diagnosis of inborn errors of BH4 metabolism. In addition, neopterin is widely used as a marker of T helper cell 1 derived cellular immune activation. BH4 deficiency can be caused by mutations in genes encoding the enzymes involved in its biosynthesis or regeneration. BH4 deficiency due to autosomal recessive mutations in BH4-metabolizing enzymes (except SR) has been described as a cause of hyperphenylalaninemia. Screening for a BH4 deficiency should be done in all newborns with high plasma phenylalanine levels. The following tests are recommended: 1) analysis of pterins in urine, 2) measurement of DHPR activity in blood from Guthrie card. Analysis of neopterin, biopterin, 5-methyl-tetrahydrofolic acid, and the neurotransmitter metabolites, 5-hydroxyindoleacetic acid and homovanillic acid, enables differentiation between severe and mild forms of BH4 deficiencies. All enzyme defects can be confirmed by enzyme activity measurement in different cells or tissue.
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