Human Biochemistry, Volume 1 - Murray R. 1993

Metabolism of Proteins and Amino Acids
Conversion of Amino Acids into Specialized Products
Tyrosine

Tyrosine is a precursor of adrenaline and noradrenaline, which are synthesized in Cells of neural origin. During The production of melanin in melanocytes and noradrenaline in Nerve Cells, DOPA serves as an intermediate; however, the hydroxylation of tyrosine in different Cell types is catalyzed by distinct Enzymes. Tyrosine hydroxylase is a copper-independent enzyme, yet, much like phenylalanine hydroxylase, it Functions with the participation of tetrahydrobiopterin. It catalyzes The formation of DOPA in Neurons and adrenal cells as an intermediate on the pathway to noradrenaline and adrenaline (Fig. 32.10). DOPA decarboxylase, a Pyridoxal phosphate-dependent enzyme, catalyzes the synthesis of dopamine. The latter undergoes further hydroxylation mediated by dopamine-ß-oxidase, a copper-containing enzyme believed to utilize Vitamin C in the reaction producing noradrenaline. In The adrenal medulla, the enzyme phenylethanolamine-N-methyltransferase uses S-adenosylmethionine to methylate the primary amino group of noradrenaline, thereby yielding adrenaline (Fig. 32.10).

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Fig. 32.10. Conversion of tyrosine to adrenaline and noradrenaline in neurons and adrenal cells. PLP – pyridoxal phosphate.

Tyrosine also serves as a precursor for the THYROID Hormones triiodothyronine and thyroxine (see Chapter 46).

Tyrosine is excreted in the urine both in its free form and as a sulfate conjugate; the formation of such conjugates is typical for most Phenolic Compounds excreted in urine.