 Further investigation of this effect revealed it to be a consequence of impaired tetrahydrobiopterin (BH4) regeneration.BH4 is a cofactor for the biopterin-dependent aromatic amino acid hydroxylase enzymes, which catalyze the conversion of phenylalanine to tyrosine, tyrosine to L-DOPA, and tryptophan to 5-HTP, oxidizing BH4 to dihydrobiopterin (BH2) in the process.BH2 must then be converted back to BH4 by the enzyme dihydropteridine reductase before it can be used again.