Hill plot

Rick Neubig RNeubig at umich.edu
Fri Jan 13 17:04:18 EST 1995

SHAUN at JASON.UTHCT.EDU ("Shaun D. Black") wrote:

>    I find this thread of real interest.  Will any of the experts 'cooperate'
> with me to answer a question?  My question is:  What else could cause a
> sigmoidal binding plot besides cooperativity?  Thanks.  Best to all,  Shaun

Hi Shaun,
It sounds like this case is one in which enzyme activity is being
monitored but to answer your question, there is at least one cause which
is an artifact.

If you do binding with a non-limiting amount of receptor and have
a radioligand that is impure you can get sigmoid binding curves when
the binding is really mass action. 

The example where I ran into this was the nicotinic acetylcholine
receptor. It was easy to have 50 nM receptor and 10 nM ligand with
a Kd of 10 nM. If the ligand was only 80-90% pure (i.e. 10-20%
had spontaneously broken down to choline and acetate) you would get
an artifactually high "free" concentration at the low concentrations
leading to a convex upward Scatchard plot. It turns out that the
binding is actually cooperative but the artifact made some data
look more cooperative than it should have been.

Rick Neubig

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