DOPA decarboxylase is a type of enzyme, a protein that helps a chemical reaction to take place. It is found in the body, where it enables the conversion of a substance called levodopa, or L-DOPA, into dopamine. L-DOPA is used as a treatment for people with Parkinson's disease, who have a lack of dopamine in the brain. As DOPA decarboxylase is found throughout the body, there is the problem that the L-DOPA given to Parkinson's patients might be converted into dopamine before it reaches the brain. In order to prevent this, what is called a DOPA decarboxylase inhibitor, a substance that stops DOPA decarboxylase from working outside the brain, is normally given together with L-DOPA.
Parkinson's disease is a disorder of the brain, where cells die off in an area called the substantia nigra, which is responsible for controlling movement. These cells would normally produce dopamine which they use to send nerve signals to muscles to generate movement. In Parkinson's disease, progressive loss of cells and an increasing deficiency of dopamine eventually cause problems such as shaking, or tremors, rigid muscles and slow movement. Most often, a tremor appears in one or both hands and is worse at rest. Walking deteriorates into shuffling, the person may occasionally freeze completely, and coordination becomes poor, resulting in falls.
The standard treatment for Parkinson's disease involves increasing the amount of dopamine available to remaining cells in the substantia nigra. Dopamine itself cannot be taken, because it is not very well absorbed by the gut and does not effectively enter the brain, the place where it is needed. L-DOPA is a better alternative because it can penetrate the brain and is also absorbed well from the gut. Inside the brain, L-DOPA is converted by the DOPA decarboxylase enzyme into dopamine, but unfortunately DOPA decarboxylase in the rest of the body also converts L-DOPA. This means that much higher doses of L-DOPA are needed to ensure enough reaches the brain, and the formation of dopamine in tissues outside the brain causes side effects, such as irregular heartbeat and nausea.
DOPA decarboxylase inhibitors, such as carbidopa, are drugs that do not enter the brain but prevent the breakdown of L-DOPA in the rest of the body. This means that a much lower dose of L-DOPA is needed to achieve the required concentration in the brain, and side effects caused by dopamine being produced outside the brain are reduced. For the treatment of Parkinson's, L-DOPA is always given in combination with a DOPA decarboxylase inhibitor. While drug treatment cannot prevent the progression of Parkinson's disease it can increase longevity and may help to control symptoms for a number of years.