Bimodal inhibition of caspase-1 by aryloxymethyl and acyloxymethyl ketones.

The caspase-1 (interleukin 1beta-converting enzyme; ICE) titrant 3-[2-(2-benzyloxycarbonylamino-3-methylbutyrylamino) propionylamino]-4 -oxo-5-(2-oxo-2H-chromen-7-yloxy)pentanoic acid (1 (Dang, L. C., et al. (1996) Biochemistry 35, 14910-14916)) inhibits caspase-1 activity rapidly, while release of the 7-hydroxycoumarin fluorophore is much slower. Progress curve analysis of 1 and of the related acyloxymethyl ketone 3-[2-(2-benzyloxycarbonylamino-3-methylbutyrylamino) propionylamino]-4 -oxo-5-(1-oxo-3-phenylpropoxy)pentanoic acid (2) identifies distinctive residual patterns which are caused by the superimposition of potent slow-binding reversible inhibition with slower, irreversible inactivation. Standard kinetic models are not entirely adequate for analysis of these bimodal inhibitors, but by measuring the kinetic properties of these inhibitors by several independent techniques and comparing these to simulations which closely mimic the inhibitor actions, careful application of the standard models can provide reasonably accurate kinetic constants.