I am doing an experiment on arginase to see if different compounds can inhibit this enzyme. I want to calculate IC50 and i reached the plateau at a inhibition of 80%. it is ok or i should increase the concentration until i reach the 100% inhibition? The plateau can be at any value?
The plateau CAN be at any value. Most typically it's at 100% inhibition, but your inhibitor could be doing something strange, and aggregating/precipitating at higher concentrations, preventing it from getting good exposure to the binding site. Similarly, an allosteric inhibitor might be binding a site that controls some, but not all, of an enzyme's activity. You could also have two binding sites, and simply be saturating one first - you'd see two different plateaus in such a case if you went higher. Or you could have some sort of impurity in your protein preparation. Or it could just be some background artifact from the assay itself - maybe the drug is interfering with the readout somehow.
I'd generally try to increase drug concentration to see if you can get closer to 100% inhibition, but a plateau lower than that is not entirely unheard of, and if you've got a good number of data points defining it (ie: more than 2), it should be acceptable to most reviewers. The real question, however, then becomes whether 80% inhibition is enough to do anything of value biologically.
The plateau CAN be at any value. Most typically it's at 100% inhibition, but your inhibitor could be doing something strange, and aggregating/precipitating at higher concentrations, preventing it from getting good exposure to the binding site. Similarly, an allosteric inhibitor might be binding a site that controls some, but not all, of an enzyme's activity. You could also have two binding sites, and simply be saturating one first - you'd see two different plateaus in such a case if you went higher. Or you could have some sort of impurity in your protein preparation. Or it could just be some background artifact from the assay itself - maybe the drug is interfering with the readout somehow.
I'd generally try to increase drug concentration to see if you can get closer to 100% inhibition, but a plateau lower than that is not entirely unheard of, and if you've got a good number of data points defining it (ie: more than 2), it should be acceptable to most reviewers. The real question, however, then becomes whether 80% inhibition is enough to do anything of value biologically.
I would point out there are many different ways a compound can act as an inhibitor. With an irreversible inhibitor you probably can get close to 100% inhibition. But with a reversible inhibitor, again it will depend upon whether it is a competitive inhibitor or not, but in most cases it is not a linear dependence but rather a curve as you asymptotically approach 100% inhibition. That is why people usually report IC50
A closer look at the mechanism involved in the inhibition and the nature of the Enzyme-Inhibitor (EI) complex will suggest whether it is possible to attain a 100% inhibition. However, the plateau can be at any value.
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