Human DOPA Decarboxylase in Monoamine Metabolism, Pharmacology, and Synucleinopathies

Human DOPA decarboxylase is the pyridoxal 5'-phosphate-dependent enzyme that produces dopamine and serotonin. For most of its history it has been treated as a constitutive downstream step rather than a point of regulation, and it has received correspondingly little mechanistic attention. The human enzyme is broadly expressed across central monoaminergic neurons and peripheral neuroendocrine tissues, where it operates in different chemical environments and under different pharmacological constraints. Structurally, it interconverts between an open, cofactor-free apo-form and a closed, catalytically competent holo-form, with conformational dynamics that extend well beyond the active site. Its activity is further tuned by cofactor availability, by phosphorylation, and by protein partners, of which the most notable is α-synuclein Its effect on the enzyme is documented in cellular systems but still unresolved at the molecular level. Pharmacologically, carbidopa is its principal peripheral inhibitor, but its mechanism remains incompletely characterized, and chronic therapy raises rather than lowers circulating AADC levels. In parallel, AADC measured in cerebrospinal fluid has emerged as one of the most discriminating fluid biomarkers for Parkinson's disease and dementia with Lewy bodies, with changes detectable years before motor onset. These different layers, structural, regulatory, pharmacological, and diagnostic, converge on a single enzyme whose biology has long been treated as settled, and which is now being reevaluated as a fundamental node in the molecular landscape of synucleinopathies.