disintegrations per minute

[biology_06er]

Hi there,

We recently carried out a radioligand binding assay and it says we must graph the DPMs along the y axis with conc. on the x-axis...just wondering what exactly DPMs are..is it the count of how much ligand is bound to the receptor

Thanks,
b_06er

[blcr11]

The dpm are the disintegrations per minute your detector sees due to the decay of the radioactive label present in the assay tube. From the sounds of it, you are being asked to plot the dpm for a standard curve? If you really need dpm (and not just the raw cpm, which is what your detector will read out) you need to know the efficiency of your detector. Say the detector efficiency is 75 %, then dpm = cpm/0.75. The dpm’s are related to the amount of substance in the assay via the specific activity of the radioactive ligand usually given in Curies/mole or some such unit, which is convertible to dpm/mole, etc. Most of the time for RIA work, though, you’re plotting the %B/F as a function of ligand concentration and that works with either cpm or dpm and you don’t typically plot things as a direct function of the dpm. Not sure, then, if you are doing RIA or something else.

[blcr11]

Should have said it the other way around: you don't normally plot dpm directly as a function of concentration of ligand. Usually you've transformed the dpm or cpm readout into some form that observes saturation phenomenon, like %B/F or %B.

[biology_06er]

Hi, thx for the reply..

Well I think i get the explanation but in our experiment we used CP55940 with the radioligand SR141716A in and not in the presence of non-hydrolysable analogue, GTPgammaS..what was seen that when the GTPgS was not present, as the concentration of CP55 decrease the CPM increased...(same with in the presence of GTP)...but I guess I am the comparing the DPM at concs. w/ and w/o the GTPgS...
EG. of my results

CONC. DPM;NO GTPgS DPM;WITH GTPgS
1.00E-6M 2653 3184
1.00E-9M 4193 5032

what I am confused about is that when there is GTPgS the SR affinity is increased right? So the more conc. of CPP the more displaced it gets hence the more displaced it gets the DPM should be higher? Does that make annnnnnnnny sense at all or am I just completely wrong?

Cheers,
b_06er

[biology_06er]

and yepp..its DPM but we were given the DPM values by the technician after the lab (as it took a while to count) so we just hv the DPM's straight out..w/ no mention of efficency......