Which is not an endogenous catecholamine?
**Core Concept**
The catecholamines are a group of endogenous compounds that play crucial roles in the body's physiological response to stress, including the "fight or flight" response. These compounds are derived from the amino acid tyrosine and include dopamine, norepinephrine, and epinephrine.
**Why the Correct Answer is Right**
The correct answer is not an endogenous catecholamine because it is not synthesized within the body from tyrosine. The endogenous catecholamines are produced through a series of enzyme-catalyzed reactions, including hydroxylation of tyrosine to L-DOPA, decarboxylation of L-DOPA to dopamine, and subsequent hydroxylation of dopamine to norepinephrine. Epinephrine is then synthesized from norepinephrine through the action of the enzyme phenylethanolamine N-methyltransferase.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because dopamine is indeed an endogenous catecholamine, synthesized from L-DOPA through the action of the enzyme aromatic L-amino acid decarboxylase.
**Option B:** This option is incorrect because norepinephrine is an endogenous catecholamine, synthesized from dopamine through the action of the enzyme dopamine beta-hydroxylase.
**Option C:** This option is incorrect because epinephrine is an endogenous catecholamine, synthesized from norepinephrine through the action of the enzyme phenylethanolamine N-methyltransferase.
**Clinical Pearl / High-Yield Fact**
The catecholamines play a crucial role in the body's response to stress, including the "fight or flight" response. This response is mediated by the sympathetic nervous system and involves the release of epinephrine from the adrenal medulla and norepinephrine from sympathetic nerve terminals.
**Correct Answer:** D.