Diversity of Secreted Messengers

Question 1

Types of Secreted Messengers: (5 Types)

Answer 1

1. Small Water-Soluble Molecules
2. Thyroid Hormones
3. Eicosanoids
4. Steroid Hormones
5. Peptide/Polypeptide Messengers

Question 2

Small Water-Soluble Molecules as Secreted Messengers

Answer 2

Acetylcholine: Acts in neuromuscular junctions, acetylcholinesterases rapidly break down ACh

Catecholamines (Made from amino acids, dopamine, noradrenaline, adrenaline): Act as both neurotransmitters and hormones. All are made from tyrosine.

Decarboxylation of amino acids produces the corresponding amines (e.g. Histidine to Histamine)
Other amino acids are used as neurotransmitters without any modifications (e.g. Glycine, Glutamate)

Question 3

Thyroid Hormones

Answer 3

Thyroxine (T4) is made in the thyroid gland from tyrosine residues located within a very large precursor protein called thyroglobulin.
Tyrosine side chains in thyroglobulin are iodinated.
T3 (more active than T4) is mainly produced by de-iodination of T4 in liver and kidney.
T3 named as it has 3 iodines, T4 as it has 4.

Question 4

Eicosanoids

Answer 4

Eicosanoids are made from unsaturated fatty acids of =>20 carbon atoms
Prostaglandins and other eicosanoids are made from arachidonic acid by pathways in which initial oxidation and cyclisation steps catalyzed by cyclo-oxygenases.
Said cyclo-oxygenases are inhibited by aspirin and ibuprofen.

Question 5

Steroid Hormones

Answer 5

Steroids are based on the sterane nucleus (4 Carbon rings).
They are made from cholesterol by cleavage of the side chain.
Cortisol is released from the adrenal gland during stress or starvation.
Dexamethasone is used medically to suppress the immune system.
Other examples: testosterone, oestradiol

Question 6

Peptide/Polypeptide Messengers

Answer 6

Made from amino acids, but differ from other secreted messengers in that they are encoded in the genome and are synthesised on ribosomes.
They vary in size from 3 to >200 amino acids
Examples of the 3 types:
1. Hormones e.g. insulin, glucagon
2. Neurotransmitters e.g. enkephalins (5 amino acids), substance P (11 amino acids)
3. Local mediators e.g. NGF, PDGF

Question 7

“Mystery of the Missing Piece in Insulin”

Answer 7

Insulin, Relaxin, Insulin-Like Growth factors (IGFs) 1 and 2.
All 4 have related regions corresponding to the A and B chains of insulin, and a highly conserved pattern of disulfide bridges. However the A and B chains of IGFs 1 and 2 are part of a longer single polypeptide (i.e. there is a missing piece on insulin and relaxin).
This missing piece is known as “C peptide”
Insulin is initially synthesised as proinsulin (A promessenger!)
Cleavage of C peptide from proinsulin produces insulin.