Ambivalence on the Multiplicity of Mammalian Aromatic L-Amino Acid Decarboxylase
Hydroxytryptophan is decarboxylated to 5-hydroxytryptamine (serotonin) by aromatic L-amino acid decarboxylase which requires pyridoxal phosphate and is widely distributed throughout mammalian tissues, occurring most abundantly in the pineal gland, liver, kidney, adrenal medulla, and striatum. Earlier studies concluded that 3,4-dihydroxyphenylalanine decarboxylase (which catalyzes the decarboxylation of dopa, producing dopamine) and 5-hydroxytryptophan decarboxylase (which catalyzes the decarboxylation of 5-hydroxytryptophan, yielding serotonin) are the same enzyme, which the IUPAC Commission on Biomedical Nomenclature in 1972 named aromatic L-amino acid decarboxylase (EC 184.108.40.206). However, recent studies have questioned the validity of a single enzyme capable of decarboxylating both substrates. For example, since the pineal gland accumulates a large concentration of serotonin, melatonin and other indoleamines, it is assumed that the enzyme functions as a 5-hydroxytryptophan decarboxylase. On the other hand, since the striatum and the adrenal medulla accumulate mainly dopamine, norepinephrine and epinephrine, it is felt that the enzyme primarily decarboxylates dopa. Other factors dealing with the complexity of catalytic process are the results of reports revealing that both dopa decarboxylase and histidine decarboxylase exhibit complete immunochemical cross reactivity, suggesting the presence of similar antigenic recognition sites.
KeywordsPineal Gland Adrenal Medulla Pyridoxal Phosphate Histidine Decarboxylase Dopa Decarboxylase
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