{{DISPLAYTITLE:L-DOPA}} -DOPA, also known as levodopa and -3,4-dihydroxyphenylalanine, is an amino acid that is made and used as part of the normal biology of some plants and animals, including humans. Humans, as well as a portion of the other animals that utilize -DOPA, make it via biosynthesis from the amino acid -tyrosine. -DOPA is the precursor to the neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline), which are collectively known as catecholamines. Furthermore, -DOPA itself mediates neurotrophic factor release by the brain and CNS. -DOPA can be manufactured and in its pure form is sold as a psychoactive drug with the INN levodopa; trade names include Sinemet, Pharmacopa, Atamet, and Stalevo. As a drug, it is used in the clinical treatment of Parkinson's disease and dopamine-responsive dystonia. -DOPA has a counterpart with opposite chirality, -DOPA. As is true for many molecules, the human body produces only one of these isomers (the -DOPA form). The enantiomeric purity of -DOPA may be analyzed by determination of the optical rotation or by chiral thin-layer chromatography. Medical use -DOPA crosses the protective blood–brain barrier, whereas dopamine itself cannot. Thus, -DOPA is used to increase dopamine concentrations in the treatment of Parkinson's disease, Parkinsonism, dopamine-responsive dystonia and Parkinson-plus syndrome. The therapeutic efficacy is different for different kinds of symptoms. Bradykinesia and rigidity are the most responsive symptoms while tremors are less responsive to levodopa therapy. Speech, swallowing disorders, postural instability and freezing gait are the least responsive symptoms. Once -DOPA has entered the central nervous system, it is converted into dopamine by the enzyme aromatic -amino acid decarboxylase, also known as DOPA decarboxylase. Pyridoxal phosphate (vitamin B6) is a required cofactor in this reaction, and may occasionally be administered along with -DOPA, usually in the form of pyridoxine. Because levodopa bypasses the enzyme tyrosine hydroxylase, the rate-limiting step in dopamine synthesis, it is much more readily converted to dopamine than tyrosine, which is normally the natural precursor for dopamine production. In humans, conversion of -DOPA to dopamine does not only occur within the central nervous system. Cells in the peripheral nervous system perform the same task. Thus administering -DOPA alone will lead to increased dopamine signaling in the periphery as well. Excessive peripheral dopamine signaling is undesirable as it causes many of the adverse side effects seen with sole L-DOPA administration. To bypass these effects, it is standard clinical practice to coadminister (with -DOPA) a peripheral