<unk>  is a cofactor in the synthesis of nitric oxide.  It is also essential in the conversion of phenylalanine to tyrosine by the enzyme  phenylalanine-4-hydroxylase; the conversion of tyrosine to L-dopa by the enzyme tyrosine hydroxylase; and conversion of tryptophan to 5-hydroxytryptophan via tryptophan hydroxylase.
Tetrahydrobiopterin
Aminopterin
Nicotinamide adenine dinucleotide
Pyridoxal phosphate
Nicotinamide adenine dinucleotide phosphate
Hydroquinone
Methaemoglobin
Beta-carotene