Aromatic L-amino acid decarboxylase

AADC catalyzes several different decarboxylation reactions:[2]

• L-DOPA to dopamine – a neurotransmitter
• L-Phenylalanine to phenethylamine – a trace amine which functions as a neuromodulator
• L-Tyrosine to tyramine – a trace amine neuromodulator
• L-Histidine to histamine – a neurotransmitter
• L-Tryptophan to tryptamine – a trace amine neuromodulator
• 5-HTP to serotonin (5-hydroxytryptamine) – a neurotransmitter

The enzyme uses pyridoxal phosphate, the active form of vitamin B₆, as a cofactor. However, some of these reactions do not seem to bear much or any biological significance. For example, histamine is biosynthesised strictly via the enzyme histidine decarboxylase in humans and other organisms.[3][4]

Biosynthetic pathways for catecholamines and trace amines in the human brain[5][6][7] L-Phenylalanine L-Tyrosine L-DOPA Epinephrine Phenethylamine p-Tyramine Dopamine Norepinephrine N-Methylphenethylamine N-Methyltyramine p-Octopamine Synephrine 3-Methoxytyramine AADC AADC AADC primary pathway PNMT PNMT PNMT PNMT AAAH AAAH brain CYP2D6 minor pathway COMT DBH DBH In humans, catecholamines and phenethylaminergic trace amines are derived from the amino acid L-phenylalanine.

Human serotonin biosynthesis pathway

In normal dopamine and serotonin (5-HT) neurotransmitter synthesis, AADC is not the rate-limiting step in either reaction. However, AADC 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). AADC 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.

In humans, AADC is also the rate-limiting enzyme in the formation of trace amines. Aromatic l-amino acid decarboxylase deficiency is associated with various symptoms as severe developmental delay, oculogyric crises and autonomic dysfunction. The molecular and clinical spectrum of AAAC deficiency is heterogeneous. The first case of AADC deficiency was described in twin brothers 1990. Patients can be treated with dopamine agonists, MAO inhibitors, and pyridoxine (vitamin B₆).[8] Clinical phenotype and response to treatment is variable and the long-term and functional outcome is unknown. To provide a basis for improving the understanding of the epidemiology, genotype–phenotype correlation and outcome of these diseases their impact on the quality of life of patients, and for evaluating diagnostic and therapeutic strategies a patient registry was established by the noncommercial International Working Group on Neurotransmitter Related Disorders (iNTD).[9]

The gene encoding the enzyme is referred to as DDC and located on chromosome 7 in humans.[10] 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[11] and autism. No direct correlation between gene variation and autism was found.[12]

• Aromatic L-amino acid decarboxylase inhibitor, a class of anti-Parkinson drugs
• Aromatic amino acids
• Histidine decarboxylase

• Aromatic-L-Amino-Acid+Decarboxylases at the US National Library of Medicine Medical Subject Headings (MeSH)