Within the last year or so, Ron Hutton was supplementing his usual meds, primarily sinemet, I think, with l-tyrosine. He found that when he went "off" while on this regimen, it was much worse than before he began tyrosine. I suggested that the l-tyrosine was actually interfering with the absorption of l-dopa by competing with it for the brain cell membrane uptake system that they share. If I remember correctly, the problem was solved when he discontinued the l-tyrosine.

l-Tyrosine is the precursor of dopamine. Two enzymatic steps inside the brain cell convert tyrosine into dopamine. If one is low on the appropriate healthy brain cells, as we PDers are, there is just not enough "machinery" to provide adequate conversion of tyrosine to dopamine. Starting out with l-dopa, the remaining cells only have to do a single step to make dopamine. I will see if I can find that thread with that exchange.
Robert

ps I went back and found the thread mentioned above. It was actually in Nov and Dec of 08.