AbstractA pigmented subclone of Cloudman S91 melanoma cells, PS1‐wild type, can grow in medium lacking tyrosine. This ability is conferred by phenylalanine hydroxylase activity, and not by tryptophan hydroxylase, tyrosine hydroxylase or tyrosinase activities, although the latter activity is also present in these cells. Conversion of phenylalanine to tyrosine was measured in living cells by chromatographic identification of the metabolites of 14Cphenylalanine and in cell extracts using a sensitive assay for phenylalanine hydroxylase. Phenylalanine hydroxylase activity in melanoma cell extracts was identified by its inhibition with p‐chlorophenylalanine and not with 6‐fluorotryptophan, 3‐iodotyrosine, phenylthiourea, tyrosine or tryptophan; and by adsorption with antiserum prepared against purified rat liver phenylalanine hydroxylase, and migration of immunoprecipitable activity with authentic phenylalanine hydroxylase subunits in sodium dodecyl sulfate‐polyacrylamide gel electr
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