Physiology of Neurotransmitters and Pigments

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Questions and Answers

What is the primary physiological role of serotonin in the body?

Control of pain and sleep (correct)

Decarboxylation of histidine

Synthesis of melanin

Regulation of immune responses

Which step is considered the rate-limiting step in the biosynthesis of catecholamines?

Degradation by monoamine oxidase

Decarboxylation of histidine

Conversion of tyrosine to DOPA (correct)

Synthesis of epinephrine

What triggers the release of epinephrine and nor-epinephrine?

High carbohydrate intake

Increased levels of serotonin

Altitude changes

Frigid temperatures and exercise (correct)

How are catecholamines inactivated in the body?

Through oxidative deamination and O-methylation Signup and view all the answers

Which cells are responsible for the formation of melanin in the skin?

Melanocytes Signup and view all the answers

What is the rate-limiting step in the biosynthesis of porphyrins?

Formation of δ-amino levulinic acid (ALA) Signup and view all the answers

Which metal ion is typically bound by porphyrins?

Fe+2 Signup and view all the answers

How many grams of heme are synthesized and destroyed daily?

6-7 gm Signup and view all the answers

What is the main site of heme biosynthesis in the human body?

Liver Signup and view all the answers

Which enzyme catalyzes the first step of heme degradation?

Heme oxidase Signup and view all the answers

What is produced when two molecules of ALA condense?

Porphobilinogen Signup and view all the answers

What substance is formed from the degradation of heme and is reduced to bilirubin?

Billiverdin Signup and view all the answers

Which of the following reactions occur in mitochondria during porphyrin biosynthesis?

Formation of δ-amino levulinic acid (ALA) Signup and view all the answers

What increases the solubility of Bilirubin in hepatocytes?

Addition of two molecules of Glucuronic acid Signup and view all the answers

What is the primary cause of hemolytic jaundice?

Increased red blood cell destruction Signup and view all the answers

Creatine phosphate serves a crucial role in maintaining the level of which molecule during intense muscular contraction?

ATP Signup and view all the answers

Which of the following is formed when Bilirubin diglucuronide is reduced by bacteria in the gut?

Urobilinogen Signup and view all the answers

What is a consequence of elevated levels of bilirubin in the bloodstream?

Toxicity to the central nervous system Signup and view all the answers

How is creatinine formed from phosphocreatine?

By spontaneous cyclization at a slow rate Signup and view all the answers

Which group of amino acids is involved in the synthesis of creatine?

Glycine and arginine Signup and view all the answers

What role does creatine kinase play in the diagnosis of myocardial infarction?

Its presence in plasma indicates heart muscle damage Signup and view all the answers

Study Notes

Nitrogen Metabolism

Nitrogen is disposed of through the urea cycle.
The carbon skeleton is broken down.
Examples of N-containing substances include:
- Porphyrins (heme)
- Creatine
- Histamine
- Serotonin
- Catecholamines
- Melanin

Porphyrins

Porphyrins are cyclic compounds that bind metal ions, typically iron (Fe2+, Fe3+).
Porphyrins are crucial components in hemoglobin, myoglobin, cytochromes, catalase, and tryptophan pyrrolase.
Heme is a porphyrin with a central iron ion coordinated in its center.
Heme is constantly synthesized and destroyed in the body (6-7 gm daily).
The ring structure consists of four pyrrole rings linked by methene bridges.
Porphyrin types (I, II, III, IV) differ in the side chains attached to the pyrrole rings.
Porphyrins contain side chains (acetate, propionate) attached to the pyrrole rings in different arrangements.

Porphyrin Biosynthesis

The liver is the primary site for heme biosynthesis.
The first and last three reactions in the cycle occur in mitochondria, while other reactions take place in the cytosol.
All carbon and nitrogen atoms in porphyrins arise from glycine and succinyl CoA.
The rate-limiting step is the formation of 8-amino levulinic acid (ALA) catalyzed by the enzyme ALA synthase.
ALA synthase is inhibited by heme.
Two molecules of ALA condense to form porphobilinogen via ALA dehydratase.

Porphyrin further biosynthesis

Four porphobilinogen molecules condense to form uroporphyrinogen III.
Uroporphyrinogen III is converted to heme via a series of decarboxylation reactions.

Heme Degradation

Red blood cells (RBCs) have a lifespan of 120 days.
Senescent RBCs are broken down, releasing heme.
Heme is degraded in macrophages (reticulo-endothelial system).
Heme oxygenase catalyzes the initial step in heme degradation.
Heme degradation releases carbon monoxide (CO), iron (Fe3+), and biliverdin.
Biliverdin is reduced to bilirubin.

Bilirubin metabolism

Bilirubin is transported in the blood bound to albumin.
Bilirubin is taken up by hepatocytes.
Hepatocytes conjugate bilirubin with glucuronic acid increasing its solubility.
Conjugated bilirubin is excreted into bile.
In the intestines, bacteria convert bilirubin diglucuronide to urobilinogen.
Some urobilinogen is reabsorbed and converted to urobilin in the kidneys, contributing to urine color.
Most urobilinogen is converted to stercobilin, contributing to the brown color of feces.

Creatine

Creatine phosphate is a high-energy compound.
Creatine phosphate stores energy in muscle cells.
Creatine phosphate can donate a phosphate group to ADP, regenerating ATP during intense muscle contractions.
Creatine kinase (CK) is an enzyme involved in this cycle.
Plasma CK levels can be used to diagnose myocardial infarction (heart attack).
Creatine is synthesized from glycine, arginine, and methionine.
Creatinine is a breakdown product of creatine, excreted in urine.
Creatinine levels can be used to estimate total body mass and as a measure of kidney function.

Histamine

Histamine is a chemical messenger involved in various bodily responses (allergy, inflammation, gastric acid secretion, neurotransmission).
Histamine is produced from histidine via decarboxylation.
Mast cells release histamine in response to allergic reactions.
Antihistamines can block histamine's effects.

Serotonin

Serotonin is synthesized from tryptophan.
It is stored in various tissues like the small intestine, platelets, and CNS.
Serotonin performs multiple physiological functions, including controlling pain, blood pressure, temperature regulation, and sleep.

Catecholamines

Catecholamines (dopamine, norepinephrine, epinephrine) are biologically active amines acting as neurotransmitters in the brain and central nervous system (CNS).
They are involved in the "fight or flight" response.
Nor-epinephrine and epinephrine are synthesized in the adrenal medulla.
Catecholamines are inactivated through oxidative deamination by enzymes like MAO and O-methylation by COMT.

Melanin

Melanin is a pigment found in various tissues (eye, hair, skin).
Melanocytes are the pigment-producing cells in the skin.
Melanin protects against harmful effects of sunlight.

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Description

Explore the intricate roles of neurotransmitters like serotonin and catecholamines, alongside the biosynthesis and degradation of heme and melanin. This quiz delves into key biochemical processes in the human body and their physiological implications. Test your understanding of these essential concepts in human physiology.