The following plot is excerpted from Bazzichi et al. Arthritis Res Ther 2004 7 R

Question

The following plot is excerpted from Bazzichi et al. Arthritis Res Ther 2004 7 R189. The data refer to the binding of PHICGS, a radiolabeled probe of the Aps adenosine receptor (AR), to cel membranes extracted from neutrophils (white blood cells) of healthy subjects (open symbols) and patients suffering from systemic sclerosis (a type of arthritis; solid symbols). 2 1,5 D 10 20 30 40 HI CGS21o Bound (fmolimg) (a) What type of plot is it? (1 mark) (b) What is the dissociation constant of [HICGs for AR? Show your work (3 marks)

Explanation / Answer

a) The above-said plot is Scatchard analysis plot( the total amount of bound ligand divided by its free concentration against the total bound ligand).In this plot, the slope represents association constant for the binding of a ligand to a receptor, and the x-intercept is equal to the number of binding sites for the ligand in a single kind of receptor.

b) The plot yields a straight line of slope -K, where K is the affinity constant for ligand binding as I said earlier. The affinity constant is the inverse of the dissociation constant

now we can see that both are the straight line.
we know that one of the st line equation is represented as (x/a)+(y/b) =1 where a and b are defined as x-intercept and y-intercept.

so from any of the st line (as they are parallel to each other there slope will always equal)

x/35 +y/1.5 = 1 or y= -0.043x + 1.5 (I have used value of binding in healthy subject)

hence affinity constant= 0.043 nM-1 , Therefore, dissociation constant = 1/0.043 = 23.26nM

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