 Queuine depletion appears to impair this "recycling" process, resulting in a deficit of BH4 and an excess of BH2, which in turn impairs the activity of the aromatic amino acid hydroxylase enzymes.Because these enzymes play a critical role in the biosynthesis of the neurotransmitters serotonin, melatonin, dopamine, norepinephrine, and epinephrine, problems with biopterin metabolism have long been considered a promising candidate to explain the origins of some psychiatric disorders characterized by imbalances in these neurotransmitters, including depression and schizophrenia.A number of studies suggest that these disorders are indeed associated with disrupted biopterin metabolism.