Posted by Larry Hoover on June 18, 2003, at 13:53:55
In reply to Re: receptor affinity » kotsunega, posted by Larry Hoover on June 18, 2003, at 9:39:43
> > > Ok-I read this a LONG time ago, but from what I remember the "numbers" that you see (with regard to "affinity") have to do indirectly with the number of molecules of a given substance it takes to bind to 50% of the receptors (I think outside the body in a dish-but it is close enough analog of what happens in the body that it is useful info).
>
> No, sorry. That's the IC50, or EC50. That's the concentration of a ligand which has the physiological effect of inhibiting or exciting 50% of the available receptors.There is a commonly used definition of Kd that is in molar units (concentration), simply the concentration at which 50% of the receptor sites are occupied.
See: http://biosci.usc.edu/courses/2001-spring/documents/bisc411-handout24.pdf
> If affinity is expressed in terms of K(subscript)d, then you're talking about a standardized term for affinity. K(subscript)d is the ratio of two opposing effects, k(subscript)d (little k), which is the dissociation rate constant, i.e. the tendency of the ligand to release its bond with the receptor as a function of time, and k(subscript)a, the association rate constant, which is the tendency of the ligand to find the receptor in the first place.This definition is supported by:
http://www.nanomedicine.com/NMI/3.5.2.htmIt seems my brain filtered out the volume term in my definition of Kd, so it is possible there is some commonality in the definitions, but the latter one involves time (rate), so there are differences, too.
Damn, I hate it when scientists can't "keep it simple".
Lar
poster:Larry Hoover
thread:234389
URL: http://www.dr-bob.org/babble/20030614/msgs/234830.html