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Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
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Thyroid hormone formation involves many steps. Which of the following amino acid is the precursor for thyroxine?
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The correct answer is  Tyrosine: Tyrosine is the precursor of triiodothyronine and thyroxine. Neural cells conve tyrosine to epinephrine and norepinephrine. While dopa is also an intermediate in the formation of melanin, different enzymes hydroxylate tyrosine in melanocytes. Dopa decarboxylase, a pyridoxal phosphate-dependent enzyme, forms dopamine. Subsequent hydroxylation by dopamine -oxidase then forms norepinephrine. In the adrenal medulla, phenylethanolamine-N-methyltransferase utilizes S-adenosylmethionine to methylate the primary amine of norepinephrine, forming epinephrine. Ref: Rodwell V.W. (2011). Chapter 30. Conversion of Amino Acids to Specialized Products. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.