Tetrahydrobiopterin is a Feedback Modulator of the Interleukin-2 Receptor Complex: Manifestations and Mechanisms

It has been well established that tetrahydrobiopterin (H4 biopterin) functions as an electron donor for the hydroxylation of the aromatic amino acids phenylalanine, tyrosine, and tryptophan [1]. More recently it has been shown that it also participates in the formation of NO − from arginine [2].