Deamination of norepinephrine and epinephrine produces an inactive acid metabolite, DHMA | Deamination of norepinephrine and epinephrine produces a reactive aldehyde that is reduced to form DHPG |
The major pathway of VMA formation is via O-methylation of DHMA formed from deamination of norepinephrine | The major pathway of VMA formation is via oxidation of MHPG formed by O-methylation of DHPG |
Catecholamine stores are static | Catecholamine stores are dynamic |
Most neuronal catecholamines are metabolized after release | Most neuronal catecholamines are metabolized intraneuronally after leakage from stores |
Neuronal catecholamine turnover depends mainly on nerve activity | Neuronal catecholamine turnover is much more dependent upon vesicular leakage than neuronal activity |
Metanephrines are formed mainly in the liver and kidneys after release of catecholamines into the circulation | Catecholamine O-methylation in the adrenal medulla is the predominant source of metanephrine and a major source of normetanephrine. |
MHPG is derived mainly from the brain | Most MHPG is derived from DHPG formed in peripheral sympathetic nerves |
MHPG sulfate reflects brain norepinephrine turnover | MHPG sulfate reflects norepinephrine turnover in the gastrointestinal tract |
Dopamine metabolites (mainly HVA) are mainly derived from the central nervous system. | Dopamine metabolites are derived mainly from peripheral non-neuronal sources, particularly the gastrointestinal tract |
Urinary dopamine is derived mainly from renal dopaminergic neurons or from plasma | Urinary dopamine is derived mainly from plasma DOPA decarboxylated in the kidney parenchyma |