Lec 2-Peptides

Question 1

Definition of neuropeptides

Answer 1

• Peptides that show activity on pharmacological test systems and are found within individual neurons

Question 2

Types of neuropeptides: Pituitary peptides

Answer 2

• ACTH (Adreno Cortico Tropic releasing Hormones)
• Growth Hormone
• Alpha-MSH (Alpha-Melanocyte Stimulating Hormone)
• Oxytocin
• Vasopressin

Question 3

Types of neuropeptides: Circulating hormones

Answer 3

• Angiotensin
• Calcitonin
• Glucagon
• Insulin

Question 4

Types of neuropeptides: Gut hormones

Answer 4

• CCK
• Gastrin
• Substance P
• VIP (Vasoactive Intenstinal Peptide)

Question 5

Types of neuropeptides: Opioid peptides

Answer 5

• Beta-endorphin
• Met-enkephalin
• Leu-enkephalin

Question 6

Types of neuropeptides: Hypothalamic releasing hormone

Answer 6

• CRF (Corticotropic Releasing Factor)
• LHRH (Luteinising Hormone Releasing Hormone)
• Somatostatin
• TRH (Thyrotropin Releasing Hormone)

Question 7

Types of neuropeptides: Miscellaneous peptides

Answer 7

• Bradykinin
• Neuropeptide Y
• Neurotensin

Question 8

Sources of peptide mediators

Answer 8

• Neuroendocrine system
• Plasma-dericed e.g. angiotensin, bradykinin
• Vascular endothelium e.g. Endothelin
• Immune system e.g. cytokines (Often >100AA)
• Growth factors e.g. EGF (Epidermal Growth Factor); NGF (Nerve Growth Factor)

Question 9

Peptide synthesis II

Answer 9

• Gene: Transcription and splicing
• mRNA: Translation
• Preprohormone: Cleavage of a signal peptide
• Prohormone: Endoproteolytic activity
• Mature peptides

Question 10

Peptide Synthesis

Answer 10

1) Synthesised first as a preprohormone

• Usually 100-250 amino acids in length
• Has the N-terminal signal sequence
• Used to inset preprohormone into the endoplasmic reticulum
• Here it is cleaved off to form Prohormone
  2) Prohormone contains
• Variable stretch (Unknown function)
• A region containing multiple copies of peptides (And/or multiple peptides)
• Active peptides excised by proteolytic enzymes (Prohormone convertases; PC1 and PC2

Question 11

Peptide synthesis

Answer 11

Question 12

Peptide Diversity: GENE SPILICNG

Answer 12

• Inclusion or exclusion of different exons allows same gene to encode more than one peptide e.g.
• Calcitonin/CGRP
• Substance P/ Substance K

Question 13

Peptide Diversity: Post-translational modification

Answer 13

• Altered modifications may alter peptide function
• Differential proteolytic cleavage can generate peptides of different length
• e.g. Prochcholecystokinin (Pro-CCK) produces at least 5 CCK peptides varying in length from 58 to 4 amino acids

Question 14

Structure of Peptides: Endogenous Peptides

Answer 14

• Usually 5-40 AAs
• Post-translationally modified (Glycosylation; Phosphorylation; Carboxylation; Acetylation)
• Very difficult to crystallise
• Structure determined by NMR
• Highly flexible: This makes synthesis ‘peptidomimetics’ very difficult

Question 15

Neuropeptides: Storage and release

Answer 15

• Intravesicular storage
• Ca-dependent release
• Often found in same terminals as classical transmitters

Question 16

Neuropeptides: Peptides

Answer 16

• Act as neuromodulators rather than fast neurotransmitters
• Often co-released with other transmitters (Co-transmission)
• Distinction between neuropeptides and hormones is fuzzy

Question 17

Neuropeptides: Peptide receptors

Answer 17

• Mainly G-protein coupled

Question 18

Inactivation of Neuropeptides

Answer 18

• Subsequent to release the activity of the peptides decays by :
• Diffusion from site

-Degradation by peptidase

• There is NO evidence for re-uptake mechanisms of the peptides

Question 19

Problems with peptides as drugs

Answer 19

• Cannot usually be given orally
  
  • Hydrolysed in gut
    
    • Eexception in cyclosporine (Contains unnatural amino acids and is not substrate for peptidase)
  • Not absorbed
• Expensive to make
• Often rapidly metabolised
• Do not cross blood-brain barrier

Question 20

Peptide receptor antagonists

Answer 20

• Very few are known
• Opiate antagonists are in clinical use
• Attempts have been made to modify known agonists
  
  • E.g. substitution of L-AAs by e.g. D-AAs (not very useful)
• Attempts to modify the peptide chain e.g. “peptoids”
  
  • Modify peptide backbone
  • Retain relative disposition of side chains
  • Difficult to maintain the affinity for a receptor
• Non-peptide antagonists may (Hopefully) be foud by random screening

Question 21

Tachykinins

Answer 21

• Substance P identified but not (Purified) in 1931
  
  • Lowers BP
  • Contracts smooth muscle
• Substance P purified in 1970- shown to belong to TACHYKININ/Neurokinin family
  
  • SP and NKA encoded by same gene
• Receptors are G-protein coupled
  
  • Selective subtypes are NK1, NK2, NK3

Question 22

Tachykinins: LOCATION

Answer 22

• Substantia nigra and corpus striatum
• Nociceptice primary afferents
• Enteric neurons

Question 23

Tachykinins: ACTIONS

Answer 23

• Contract smooth muscle e.g. GI, airways
• Mixed action on blood vessels
• Activate mast cells to release histamine
• Stimulate exocrine gland secretion

Question 24

Tachykinins: POTENT ANTAGONISTS

Answer 24

• Spantide acts on NK1 receptors
• CP96345 (Non-peptide)- random screening

Question 25

Opioid peptides

Answer 25

• 1975 Hughes and Kosterlitz
  
  • Isolated 2 brain pentapeptides (Met- and Leu- enkephalines) that had morphine- like properties
  • Endogenous ‘morphine’
• Now known that opioid peptides are encoded by 3 distinct genes
  
  • Pre-pro-opiomelanocortin (POMC)
  • Pre-pre-enkephalin
  • Pre-pro-dynorphin

Question 26

Opioids: Three main families of endogenous opioids

Answer 26

• Endorphins
• Enkephalins
• Dynorphins

Question 27

Opioids: Analgesic and cellular inhibitory effects

Answer 27

• Released in response to pain

Question 28

Opioids: Opioid peptides act on three distinct receptor subtypes

Answer 28

• sigma, kappa, Mu
• All are G-protein coupled receptors