Box 4.41Catecholamine metabolism - diagnosis of pheochromocytoma

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  • Small quantities of free norepinephrine (~0.5%) or conjugated with sulfate (~2%) are excreted in the urine
  • Catechol O-methyl transferase (COMT) converts the catecholamines to metanephrine and normetanephrine forming about 3% of total excretion
  • Monoamine oxidase (MAO) produces aldehydes that are immediately metabolized to the corresponding carboxylic acid or alcohol by aldehyde or alcohol dehydrogenases
  • MAO also catalyzes the metabolism of metanephrine and normetanephrine to vanilyl mandelic acid (VMA, ~65% of excretion) and the corresponding alcohol (MOPG, ~35% of excretion)

Clinical importance

  • The majority of DOPG comes from metabolism of neuronal norepinephrine that has not been released at synapses. Norepinephrine released into the circulation is not converted to DOPG. Thus, estimates of the excretion of non-metabolized catecholamines (i.e. epinephrine, metanephrine, norepine-phrine and normetanephrine) form a better diagnostic test for pheochromo-cytomas. Measurement of the ratio of DOPG to norepinephrine concentrations in blood may be a more sensitive way of detecting pheochromocytomas
  • Assays for catecholamines use HPLC with electrochemical detection. This separates the different catecholamines and their metabolites according to their chemical structure
  • Drugs that inhibit MAO have potentially disastrous effects on catecholamine metabolism, causing marked hypertension

From: Chapter 4, The adrenal gland

Cover of Endocrinology
Endocrinology: An Integrated Approach.
Nussey S, Whitehead S.
Copyright © 2001, BIOS Scientific Publishers Limited.

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