tetrahydrobiopterin is a cofactor of several enzymes, including the aromatic amino acid hydroxylases tryptophan 5-hydroxylase 1, tyrosine hydroxylase, and 1680529
) and regulation of human melanogenesis (8128228
). Deffciency causes severe neurological disorders characterized by hyperphenylalaninaemia and monoamine neurotransmitter deffciency, and is also implicated in Parkinson's disease, Alzheimer's disease and depression ((10727395
). tetrahydrobiopterin is present in probably every cell or tissue (10727395
). About This Pathway
While most of the tetrahydrobiopterin in mammalian cells is probably formed as described in the pathway tetrahydrobiopterin biosynthesis I, a study performed with rat brains has shown that when the activity of the enzyme sepiapterin reductase (L-threo-7,8-dihydrobiopterin forming) is limiting, a second pathway can lead to tetrahydrobiopterin production (2656673
). In this route, 6-pyruvoyl tetrahydropterin is converted by the enzyme 6-pyruvoyltetrahydropterin 2'-reductase to 6-lactoyl-5,6,7,8-tetrahydropterin, which is then converted to tetrahydrobiopterin by sepiapterin reductase (L-threo-7,8-dihydrobiopterin forming).