since tyrosine hydroxylase or tryptophan hydroxylase is the rate-limiting enzyme for the biosynthesis of catecholamines ( dopamine , norepinephrine and epinephrine ) or serotonin in monoaminergic neurons , biosynthesis of tetrahydrobiopterin from gtp may also regulate the tissue level of monoamine transmitters .
C0007412
catecholamines
C0013030
dopamine