when both dopa and other amino acids are infused IV, is that the presence of elevated levels of phenylalanine and the large aliphatic amino acids isoleucine and leucine in plasma are preventing transport of plasma dopa into the brain. This is because they share a common molecular transport system and are competing with one another to pass through. I would guess that tyrosine and tryptophan, amino acids with large, aromatic "side chains" would have the same effect if present at similar levels. I seem to remember that all five of these amino acids, including dopa, use the same or overlapping transport systems, unlike the other amino acids like glycine and alanine which enter the brain via different transport systems. In typical dietary proteins, tyrosine and tryptophan are present in lower amounts than phenylalanine, leucine and isoleucine, and would therefore provide less competention for a common transport system.

Think of the presence of separate turnstiles or revolving doors constructed to allow only people with certain body shapes or sizes to pass through. So, if there were only one or two dopa-shaped people and several hundred phenylalanine-, leucine-, isoleucine-, and ten or twenty tyrosine- and tryptophan-shaped people all simultaneously trying to get through the same turnstile, the dopa-shaped people would get in quicker after the crowd of other amino acid folks thinned out.

However, that explanation alone does not seem to fully answer the mysterious ons and offs that folks experience even in the absence of oral dopa intake. It is well-known that dopa is made in other parts of the body, especially the adrenal medulla, as well as in non-brain dopaminergic neurons. It is my understanding that the dopa made in these cells can not get out into the general body circulation where it might otherwise supply some amount of "rescue" dopa to account for an improved pre-off condition. The gradual production of dopa by the few remaining dopaminergic neurons while in the unmedicated state, as suggested by rick and others, seems to me to better account for that phenomenon.

Robert