Tyrosine is an immediate precursor of the dopamine family
of hormones. Dopamines are synthesized the adrenal medulla and central nervous
system, and regulated central and peripheral nervous system activity. These
hormones include adrenaline and noradrenaline
The major route of degradation of tyrosine is the
conversion to parahydroxyphenylpyruvate. This compound is then further degraded
by an enzyme called dioxygenase to homogentinsic acid. One of the first inborn
errors of metabolism to be recognized was called alkaptonuria, which was a lack
of dioxygenase resulting in increased spill in the urine of
parahydroxyphenylpyruvate. This condition is easily identified by blackening of
the urine upon standing.
Tyrosine is also metabolized to the thyroid hormones
thyroxin and triiodothyronine. The thyroid gland is rich in iodide. This reacts
under the influence of a peroxidase enzyme to iodinate tyrosine to form the
active thyroid hormones.
Tyrosine is also metabolized to catechol derivatives,
which may play important roles as neurotransmitters. They are also the
precursors of the tanning pigment in skin and the black pigment in hair called
melanin. The route of formation of the catecholamines is through change of
tyrosine into tyramine, and the subsequent conversion to dopa. This compound is
effective in treatment of Parkinson's disease. The debilitating symptoms of
this disease are thought to be a result of the lack of dopamine in certain
regions of the brain. Conversion of dopamine by way of vitamin C and a
copper-containing enzyme produces noradrenaline. Methylation of this compound
produces adrenaline, and important hormone and neurotransmitter.
The metabolic breakdown of adrenaline and noradrenaline
occur by way of an enzyme called monamine oxidase (MAO), with the ultimate
excretory product being vanellic acid which is excreted in the urine. The
second breakdown route for these neurotransmitters is by way of catecholamine
O-methyl transferase (COMT), a very active enzyme in neural tissues.
Noradrenaline and adrenaline are related to the proper metabolism of tyrosine