Judith Guion refers to the advice given by a friend that the amino
acid precursor of DOPA, tyrosine, might be substituted for L-DOPA in
treating PD. The answer is clearly NO for the folllwing reasons: 1) Tyrosine
hydroxylation that occurs in nerve cells is a rate-limiting step in DOPA and
dopamine biosynthesis. Increasing tyrosine in the diet is not likely to
increase its uptake and conversion to DOPA. However, this is complicated by
dietary factors, such as meat intake. 2)tyrosine uptake from the gut and
into cells is competitive with (many) other amino acids. That would not
result in a much greater supply for dopamine biosynthesis and may foul up
the delicate control mechanisms in sympathetic and dopamine equilibrium.
Tyrosine, under some conditions, may actually decrease the supply of L-DOPA
by competing with L-DOPA for transport into cells and and consequently
worsen PD. 3) The vast majority of the administered tyrosine would be
metabolized to inactive products including melanin. (No, tyrosine is not
known to induce melanoma)., a I hope this information will be useful. If
you want more see: Chapter 6 from Goodman & Gilman' Pharmacological Basis of
Therapeutics (9th) edition, 1996. Best wishes,
Steven Mayer e-mail:[log in to unmask]
Steven E. Mayer, Ph.D.
