Enzyme Inhibition in relation to Aspirin I see that aspirin (in part) works by inhibiting cycloxygenase isoenzymes and that this inhibiting is irreversible. I've had a few classes mentioning this topic in passing, but never with depth. I guess my question regards the permanence of the word "irreversible." I assume enzymes have a life span, so to speak, and that the same isoenzymes both die and are being replenished at some given rate. Am I correct in thinking that the cycloxygenase enzyme is permanently inhibited until its death (catabolism)?

‘Reversible’ and ‘Irreversible’ are standard designations in enzyme kinetics. It may be that your instructor was using the word in this sense.

As already commented, _irreversible inhibitors_ bind the enzyme in such a way that they don't dissociate from it. Either they form a covalent bond or their affinity for the binding site is extremely high. _Reversible inhibition_ is generally found with competitive inhibitors — those that resemble substrate enough to bind to the active site and compete with the substrate for it. Unless they have an extremely high affinity there will be an equilibrium between bound and unbound inhibitor, and the inhibitor can be competed out by substrate — i.e. the binding is reversible.

You can read about this in more detail, but still at an introductory level, in the Wikipedia article on Enzyme Inhibitors or this section in the online version of Berg _et al._