40 - Tetrahydrobiopterin (THB)

*Tetrahydrobiopterin (THB, BH4; Kuvan) or sapropterin, is a naturally occurring nutrient and essential cofactor of the three aromatic amino acid hydroxylase enzymes, used in the biosynthesis of the neurotransmitters serotonin (5-hydroxytryptamine (5-HT)), melatonin, dopamine, norepinephrine (noradrenaline), epinephrine (adrenaline), and nitric oxide (NO).

*THB was discovered to play a role as an enzymatic cofactor by Seymour Kaufman. The first enzyme found to use THB is phenylalanine hydroxylase (PAH).

*THB is biosynthesized from guanosine triphosphate (GTP) by three chemical reactions, those of which are mediated by the enzymes (GTP cyclohydrolase I (GTPCH), 6-pyruvoyltetrahydropterin synthase (PTPS), and sepiapterin reductase (SPR).

*THB has the following responsibilities as a cofactor:

- Tryptophan hydroxylase (TPH) for the conversion of L-tryptophan (TRP) to 5-hydroxytryptophan (5-HTP)
- Phenylalanine hydroxylase (PAH) for conversion of L-phenylalanine (PHE) to L-tyrosine (TYR)
- Tyrosine hydroxylase (TH) for the conversion of L-tyrosine to L-DOPA (DOPA)
- Nitric oxide synthase (NOS) for conversion of a guanidino nitrogen of L-arginine (L-Arg) to nitric oxide (NO)
- Glyceryl ether monooxygenase (GEMO) for the conversion of 1-alkyl-sn-glycerol to 1-hydroxyalkyl-sn-glycerol.

*A deficit in THB biosynthesis and/or regeneration results in phenylketonuria (PKU) from excess L-phenylalanine concentrations or hyperphenylalaninemia (HPA), as well as monoamine and nitric oxide neurotransmitter deficiency or chemical imbalance. The chronic presence of PKU can result in severe brain damage, including symptoms of mental retardation, microcephaly, speech impediments such as stuttering, slurring, and lisps, seizures or convulsions, and behavioral abnormalities, among other effects.

*THB, developed by BioMarin under the brand name Kuvan and approved by the United States (U.S.) Food and Drug Administration (FDA) on December 13, 2007, is a synthetic preparation of the dihydrochloride salt of the substance, used in the treatment of PKU. HPA due to THB deficiency-mediated PKU.

*According to researchers at the University of Illinois at Chicago, THB may be of benefit in the management and treatment of intractable systolic heart failure based on research in mice. This is especially significant given that THB has already been tested and determined to be safe in human patients using it in the management of phenylketonuria, and also because there are far fewer ways to manage systolic heart failure than for diastolic heart failure.