Makes sense to me. The cells are there but they aren't doing enough. Then we take Sinemet-ie. dopamine which is the end result of this enzymatic cycle so the body senses that we don't need to produce as much of the enzyme and 'round we go....This brings another question to mind. Aren't enzymes protein based? Why would proteins block the uptake of the Sinemet. Can we help replace these enzymes by taking amino acids? Time to get out the chemistry books...