Levodopa (or L-dopa), the drug first introduced into public awareness through the movie AWAKENINGS, still remains the most common drug treatment for the symptoms of PD. The name derives from L-3,4-dihydroxyphenylalanine, a neutral amino acid found in both plants and animals. Once ingested and absorbed into the body, L-dopa does not bind to plasma proteins. Dopamine cannot be introduced directly into the bloodstream because dopamine cannot cross the blood-brain barrier. L-dopa can cross through the same barrier; however, there are a few complications. Once inside the body, L-dopa is broken down by the enzymes peripheral dopa-decarboxylase (DDC) and cathechol-O-methyltransferase (COMT) into dopamine. What one wants, though, is some way of delaying the breaking down of L-dopa into dopamine until after it has passed through the blood-brain barrier. This is done with DDC inhibitors and COMT inhibitors. That is why L-dopa is usually administered alongside DDC inhibitors carbidopa (which reduces the amount of L-dopa needed by 75 percent) or benserazide because they help delay the conversion of L-dopa into dopamine in the body until the drug has made its way into the brain. Although L-dopa is effective in about 75 percent of all Parkinson's patients to the point that it can totally mask the symptoms of PD, it does not stop or diminish the degeneration of the nigrostriatal dopamine nerve terminals in the brain.
I hope you find this summary helpful. If you want to read more, I suggest visiting here and here. An excellent online article about the process of converting L-dopa into dopamine in the brain can be found here.
We also have an interesting article discussing some of the natural sources of the Parkinson's Medication, Levodopa.
