L3 - Receptor Theory 1- Drug Affinity

Question 1

What is affinity?

Answer 1

A chemical property that defines how well 2 molecules can come together

Question 2

What is a Forward reaction?

Answer 2

When a drug and agonist come together

Question 3

What is a Backward reaction?

Answer 3

When a drug and agonist come apart

Question 4

What is occupation? and what is it governed by

Answer 4

The number of cells that are occupied, governed by affinity

Question 5

What is activation? and what is it governed by

Answer 5

The “turning on” of a receptor governed by efficiacy

Question 6

How is forward rate of reaction measured?

Answer 6

K+1 [A] * [R]

Question 7

How is reverse rate of reation measured?

Answer 7

K-1 [A] [R]

Question 8

What is the equation at equilibrium?

Answer 8

K+1 [Aeq] x [Req] = K-1 [Aeq Req]

Question 9

What is Kd?

Answer 9

direct measure of affiinity a constant that defines the affinity of a receptor
(where D= dissociation)

Question 10

How is Kd measured?

Answer 10

K+1/K-1 = [Aeq] x [Req] / [Aeq Req]

Question 11

When is the Kd at a low number?

Answer 11

At high affinity

Question 12

How is occupancy measured?

Answer 12

number of receptors occupied divided by the total number of receptors
Varies between 0 (no drug present) and 1 (all receptors occupied)

Question 13

How do we practically consider tissue and incubation for measuring receptor affinity?

Answer 13

Tissue - selected to condition the recognition sites and receptors, can be isolated
Incubation - temperature must be kept cold so enzymes arent active

Question 14

How do we practically consider the radioligand for measuring receptor affinity?

Answer 14

molecular structure of the ligand must not be changed as the ligand must be active
purity - ligand must be chemically pure
degradation - solved by avoiding light, storing and low
temps and antioxidants
labelling - labelling with radioactivity must achieve
specific activity, to allow low tracer
concentrations

Question 15

Some advantages and disadvantages of H3 as a radio label

Answer 15

Advantages - indistingushable from native compounds
- long half life
- good stability
Disadvantages - specialised labs required
- expensive and difficilt to label

Question 16

Some advantages and disadvantages of I125 as a radio label

Answer 16

Advantages - if compound has an aromatic hydroxly
group (eg tryosine residues) it ca be
incorporated at high specific activities.
- iodonation very easy and cheap.
Disadvantages - easily degraded
- biological activity of the logand can
be reduced.
- Short half life.

Question 17

How do we practically consider separating bound from free for measuring receptor affinity?

Answer 17

• Usually done by filtration or centrifugation
• Issues is rate of dissociation of ligand-receptor complex, speed of separation must be compatible with affinity of ligand for receptor.
• lower affinity or higher Kd requires more efficient separation.

Question 18

How do we practically consider non-specific binding for measuring receptor affinity?

Answer 18

• lignad may also bind to other things in the test tube or other proteins in the separtation.
• can be reduced by measureing the proportion of specific to non-specific binding. (rinsing only removes unbound radioligand from incubation medium)

Question 19

What is the shape of radioligand binding curves?

Answer 19

Rectangular hyperbola

Question 20

What does the lagmuir equation meaure and what is it?

Answer 20

Describes the relationship between receptor occpancy, affinity and drug concentration.
Bound = Bmax x Xa / (Xa + Kd)

Question 21

What does transformation of the langmuir equation give?

Answer 21

Scatchard Equation - Commonly used for analysis of binding data

Question 22

Why do we need to measure receptors?

Answer 22

• To define them
• to understand the rate of drug/ligand action
• to find how many receptors are ina given tissue
• to find how receptor numbers change in disease