Dopamine, adrenaline and noradrenaline

Dopamine hydroxylase converts dopamine to norepinephrine

Deficiency of this enzyme ccauses a rare form of dysautonomia characterized by ptosis, orthostatic hypotension, hypoglycemia, and hypothermia

Hormone inactivation is mediated by two enzymes:

1. Catechol-O-methyltransferase (COMT), as its name implies, transfers a methyl group onto the hormone and is responsible for forming the metabolites metadrenaline and normetadrenaline.
2. Monoamine oxidase (MAO) removes the amine group.

The combined effects of COMT and MAO produce hydroxymethoxymandelic acid (HMMA) (formerly known as vanillyl-mandelic acid (VMA)).

🩺 Clinical Correlate

• Metadrenaline, normetadrenaline and HMMA are used as markers for pheochromocytomas and other disorders of excessive catecholamine secretion.
• Dopa decarboxylase inhibitors are used in combination with levodopa (L-dopa) in the treatment of Parkinson's disease. (See: Parkinsonism - Disease and Parkinsonism - Treatment) The dopa decarboxylase inhibitors reduce side effects by preventing the peripheral formation of dopamine from L-dopa.