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BMS 242 - Core Phys/pharm > Receptor theory II > Flashcards

Flashcards in Receptor theory II Deck (47)
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1

What is the binding of an agonist governed by?

The rate of the forwards and backwards reaction of binding

Affinity - balance between the 2

2

What is affinity and what it a balance of?

A measure of how well the drug binds to its receptor

The balance between the forwards and backwards reaction
(high affinity, high forwards reaction and slow reverse reaction)

3

What is occupancy a measure of and what is it dependant on

How many receptors are occupied by a drug at any moment in time

Dependant on AFFINITY for the drug

4

What is an antagonist?

A drug which binds to a receptor, without switching on receptor signalling - doesn't directly

5

What drugs does affinity apply to?

Equally applies to agonists and antagonists

6

What is affinity constant for?

A specific drug with its specific receptor in ANY tissue

7

What does efficacy govern?

Activation
The change in the cell, by causing the stabilisation of the active agonist/receptor configuration

8

What does efficacy apply to?

Agonists only

9

On what scale is efficacy measures?

On a scale of 0-1

High efficacy is a value of 1

10

In the case of an agonist, how is affinity related to efficacy?

They are in dependant of each-other

Reactions can have a high affinity and a low efficacy and vice versa

11

What is the efficacy of an antagonist?

0 - Unable to induce an active configuration

12

What is occupancy a measure of?

The proportion of receptors occupied with a certain drug concentration

13

On what scale is occupancy measured?

0-1

1 = all receptors occupied
0 =no receptors occupied

14

What is occupancy NOT proportional to?

Why?

The size of the response

Because the maximum response of a tissue can occur with less than 100% occupancy - the tissue can have spare receptors

Maximum response of a tissue can occur at <5% occupancy

15

Why is measuring the response not a good way to measure occupancy in order to calculate affinity?

Size of the response is governed by both:
- Affinity
- Efficacy in agonists

(not just affinity)

16

What assay is used to measure occupancy to get an idea of affinity?

Radio ligand binding assay

17

What does a radio ligand binding assay measure?

Biding of a ligand (agonist or antagonist) to a protein target

Measure affinity

18

What is the main negative point of a radio ligand binding assay?

How is this overcome?

NONE-SPECIFIC BINDING occurs

- Overcome by doing 2 experiments in parallel
- Slowly increase concentration of ligand in each tube
- Had a HUGE excess to one of the tubes
- Displaces the specific binding and adds to the non-specific binding

Then:
Tube 1 - Radio active drug is specific and non-specific

Tube 2- Radio active drug is ONLY NON-SPECIFIC

SO tube 1 -tube 2 is specific binding of the radioactive drug

19

When choosing the radio-ligand, what must be considered?

- Purity (consider isomers)
- Degradation
- How to label drug

20

What 4 steps can be taken to minimise the degradation of the radioactive drug?

1) Low temperature

2) Add oxidant (minimise oxidation)

3) Avoid light (can cause chemical changes)

4) Add free-radical scavengers

21

Example of a free-radical scavenger?

Ethanol

22

Example of an antioxidant?

Asorbic acid

23

What are 3 advantages of Tridium as a radioactive substance?

- Doesn't alter properties

- High specific activities

- Long half-life (good stability)

24

What are 3 disadvantages of Tridium as a radioactive substance?

- Dangerous to humans

- Specialist labs needed (expensive)

- Amount of label per molecule differs

25

What are the 2 advantages of using radioactive iodine as a radioactive substance?

- Easy and cheap

- Can be incorporated at high specific activities

26

What are the 2 disadvantages of using radioactive iodine as a radioactive substance?

- Short half-life

- Biological activity of the ligand measured can be reduced

27

Why must the tissue be incubated?

When blend cells to release receptors - breaks up lysosomes and releases enzymes

- Incubating prevents enzymes from breaking down the proteins and receptors

28

How separate bound ligand (specific or non-specific) from non bound?

What are the problems with this method?

Centrifugation - Forms a pellet containing the protein

Problems:
- Specific binding may occur to the pellet (must rinse)

All these steps take time:
- If the drug has a low affinity, off rate is quick
- When washing pellet, may wash off some previously bound ligand

29

What is Kd and how does it relate to affinity?

Kd - binding coefficient

The higher the Kd, the lower the affinity
Weaker the ligand is bound

30

What is the shape of the non-specific binding curve?

Linear