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Hormones - Synthesis, Transport, and Effects
Overview
Three classes of hormones:
Peptides and proteins (P&P)
Steroids (S)
Amines (A)
– Catecholamines (C)
– Thyroid hormones (T)
4 hormone physiology features:
Synthesis
Hormones can be made in advance and stored prior to secretion; the peptide, protein,, and amine hormones follow this model.
Hormones can be synthesized and secreted on demand; steroids follow this model.
Transport
This generally depends on chemical structure.
Hormones can dissolve and travel freely in the blood – the peptides and proteins and also the catecholamines: they are water soluble (aka, hydrophilic)
Hormones can bind carrier proteins – the steroid and thyroid hormones are less water soluble and travel bound to carrier proteins.
Receptor binding
The chemical relationship between hormones and target cell membranes determines whether hormones:
Bind surface membrane receptors – the peptide and protein hormones and the catecholamines, and in some cases, steroid hormones where they have non-genomic effects on the cell.
Bind intracellular receptors -- because they are lipophilic (lipid soluble), steroid hormones and thyroid hormones readily slip past the cell membrane to bind with cytoplasmic and/or nuclear receptors.
Mechanism of Action
Hormones can modify existing proteins within a cell – the peptide and protein hormones and the catecholamines.
Hormones can trigger protein synthesis – the peptide and protein hormones do this, as well, as do the steroid and thyroid hormones.
Comments on the Amines
The peptides and proteins almost NEVER behave like the steroids.
The amines divide into the catecholamines and thyroid hormones:
– Catecholamines act most like the peptides and proteins.
– The thyroid hormones act most like the steroids.
Peptide and protein hormones
Synthesis:
Peptide and protein hormones are synthesized and then stored in secretory vesicles.
First, within the nucleus of a cell, the gene for a hormone is transcribed as mRNA.
The mRNA moves to the ribosomes, where it is translated to create a preprohormone.
The preprohormone moves to the endoplasmic reticulum, where it is converted to a prohormone.
Finally, the prohormone is transported to the golgi apparatus to be packaged into secretory vesicles; within these vesicles, peptide cleavage produces the final hormone product.
The hormone is stored until its release is triggered.
Transport:
Protein and peptide hormones travel freely in the blood; recall that this is because it is water soluble, and readily dissolves.
Mechanisms of Action:
Peptide hormone binds cell membrane surface receptor and the hormone-receptor complex activates second messenger systems to initiate protein modification and synthesis.
Steroid hormones
Synthesis:
Steroid hormones are synthesized in the endoplasmic reticulum and secreted on demand; they are not stored in the cell.
Cholesterol is the parent of steroid hormones.
In the adrenal cortex, Mineralocorticoids, Glucocorticoids, or Androgens are produced.
In the testes and ovaries, aka, the gonads, testosterone and estrogen are produced.
DHEA (Dehydroepiandrosterone) and Progesterone are important intermediate steroids.
Transport:
Steroid hormones travel in the blood bound to carrier proteins. Only a small portion of steroid hormones travel freely, or unbound.
Mechanisms:
When steroid hormones bind intracellular receptors, they activate or repress transcription.
Testosterone can pass through the cell membrane to bind with these intracellular receptors.
When steroid hormones bind surface membrane receptors, they initiate non-genomic effects via second messenger systems are activated.