Further studies on tryptophan hydroxylase in rat brainstem and beef pineal

Soluble tryptophan hydroxylating enzymes have been obtained from both the beef pineal gland and rat brainstem. The enzymes have a pH optimum of 7.5 and appear to be typical aromatic ring hydroxylases. Although aniron requirement of the enzymes can be demonstrated by iron chelators, exogenous iron is not absolutely required for activity. Tetrahydrobiopterin was more effective than 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8 tetrahydropteridine as a cofactor for the enzymes. The beef pineal enzyme hydroxylates phenylalanine at approximately the same rate as it does tryptophan, whereas the brain enzyme showed no detectable activity toward phenylalanine. Finally, catechol compounds are effective inhibitors of the enzymes, presumably because of their ability to chelate iron.