Baldwin:
You have asked very complex questions about dopamine regulation,
that dont have simple answers. I would suggest that you read "Biochemical
Pathology & Pharmacology of Parkinsons Disease" by James Bennett (univ. of
Virginia). This is
chapter three in the book "The comprehensive Management of Parkinsons Disease
"
edited by M. Stern & H. Hurtig-PMA Publishing Corp., New York You will find
lots of
references at end of chapter to take you further -if desired.
Administration of tyrosine does not appear to affect parkinsonian
symptoms,
presumably because the depletion of tyrosine hydroxylase limits the ability
of tyrosine to enhance dopamine synthesis (Hornykiewicz, 1974}-The Mechanisms
of action of L-DOPA in Parkinsons Disease-Life Sci 1974; 1249-59.
Administration of levodopa, which is transported into the brain by the same
transport system as tyrosine, bypasses this rate limiting step in dopamine
synthesis. in the brain, levodopa is decarboxylated by aromatic amino acid
decarboxylase to dopamine.
Although the decarboxylase is also markedly reduced in the parkinsonian
brain, conc-entrations appear to be adequate to decarboxylate either
endogenously or exogenously produced levodopa.This paragraph lifted from "100
Maxims in Neurology"-Parkinsons Disease by John Nutt et al-Mosby Year
Book-Chapter 8
Hope this helps some, Don Conley
