Tryptophan 5-hydroxylase was partially purified from rat small intestine and characterized. The enzyme activity was mainly localized in the distal one-fourth of the small intestine. The enzyme required Fe2+, 2-amino-4-hydroxy-6,7-dimethyl-5,6,7,8-tetrahydropteridine and oxygen for full activity. The pH optimum of the reaction was 8.0. The hydroxylation rate of d-tryptophan by the enzyme was one-third that of l-tryptophan. l-Phenylalanine and l-tyrosine could not serve as substrates. The physiological significance of the enzyme is discussed.

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