Aromatic L-amino acid decarboxylase

Aromatic L-amino acid decarboxylase (, synonyms: DOPA decarboxylase, tryptophan decarboxylase, 5-hydroxytryptophan decarboxylase, AAAD) is a lyase enzyme.

Reactions
It catalyzes several different decarboxylation reactions:
 * L-DOPA to dopamine - a neurotransmitter
 * 5-HTP to serotonin (5-HT) - also a neurotransmitter
 * tryptophan to tryptamine - a precursor to many alkaloids found in plants and animals

The enzyme uses pyridoxal phosphate, the active form of vitamin B6, as a cofactor.

As a rate-limiting step
In normal dopamine and serotonin (5-HT) neurotransmitter synthesis, AAAD is not the rate-limiting step in either reaction. However, AAAD becomes the rate-limiting step of dopamine synthesis in patients treated with L-DOPA (such as in Parkinson's Disease), and the rate-limiting step of serotonin synthesis in people treated with 5-HTP (such as in mild depression or dysthymia). AAAD is inhibited by Carbidopa outside of the blood brain barrier to inhibit the premature conversion of L-DOPA to Dopamine in the treatment of Parkinson's.

AAAD is the rate-limiting enzyme in the formation of biogenic trace amines.



Genetics
The gene encoding the enzyme is referred to as DDC and located on chromosome 7 in humans. Single nucleotide polymorphisms and other gene variations have been investigated in relation to neuropsychiatric disorders, e.g., a one-base pair deletion at –601 and a four-base pair deletion at 722–725 in exon 1 in relation to bipolar disorder and autism.