dopamine beta-hydroxylase

**Biochemical pathway:** Phenylalanine → (phenylalanine hydroxylase, BH4) Tyrosine → (tyrosine hydroxylase, BH4; RATE-LIMITING) L-DOPA → (DOPA decarboxylase, B6/PLP) Dopamine → (dopamine beta-hydroxylase, Cu2+, vitamin C, O2) Norepinephrine → (PNMT, SAM; induced by cortisol) Epinephrine.

**Location:** DBH is in the membrane/lumen of synaptic (chromaffin) vesicles. Dopamine is made in the cytosol and transported into vesicles by VMAT, where DBH converts it to NE. Reserpine blocks VMAT, so dopamine and NE are depleted and cannot be stored. DBH is co-released with NE on exocytosis, and serum DBH has been used as a marker of sympathetic activity.

**Cofactors:** Copper (Cu2+ at the active site) and ascorbic acid (vitamin C) as the electron donor. Molecular oxygen is also required, so it is a monooxygenase.

**Clinical correlates:** - **DBH deficiency (congenital):** Rare autosomal recessive disorder. Patients have severe orthostatic hypotension, ptosis, hypoglycemia, and exercise intolerance. Plasma shows high dopamine and undetectable NE and epinephrine. Treat with droxidopa (L-DOPS), which is converted directly to NE by DOPA decarboxylase and bypasses DBH. - **Menkes disease:** X-linked recessive defect in ATP7A (copper transporter) causing copper deficiency, so DBH (and lysyl oxidase) dysfunction. Features are kinky/steely hair, hypotonia, seizures, neurodegeneration, and connective tissue abnormalities. Neurochemically there is high dopamine and low NE. - **Vitamin C deficiency (scurvy):** Reduces DBH activity, though the scurvy presentation is dominated by impaired collagen hydroxylation. - **Pharmacology:** Disulfiram inhibits DBH (in addition to aldehyde dehydrogenase) by chelating copper, which can raise dopamine and lower NE. This may contribute to psychosis in susceptible patients. Nepicastat is an investigational DBH inhibitor. Reserpine depletes NE indirectly by blocking VMAT. - **Pheochromocytoma/neuroblastoma:** These tumors produce catecholamines. Tumors with low DBH activity may secrete mostly dopamine.

**High-yield pearls:** - Tyrosine hydroxylase is the rate-limiting enzyme. DBH is not. - DBH is the only enzyme in the pathway inside vesicles. - The conversion of dopamine to NE is a beta-hydroxylation (adds an OH on the beta carbon of the side chain). - Cortisol from the adrenal cortex drains through the adrenal medulla and upregulates PNMT (NE → epinephrine), not DBH. - Degradation of catecholamines is by MAO and COMT, giving metabolites such as VMA and HVA (dopamine).