Because of the fact that changes in cellular morphology and internucleosomal DNA cleavage associated with apo-ptosis are events that reportedly depend upon the activity of caspases, we next employed substrate cleavage assays to assess a role for potassium in the regulation of caspases during apoptosis of oocytes and granulosa cells. We observed that KCl, NaCl, and LiCl effectively blocked caspase-1-mediated processing of pro-IL1p and caspase-3-catalyzed cleavage of cellular actin. These data differ from those of Hughes et al. using thymocytes and those of Bortner et al. using S49-Neo lymphoma cells; those studies showed that potassium prevented procaspase processing (i.e., generation of the active enzyme) but did not inhibit active caspases, as assessed by cleavage of a tetra-peptide (DEVD) known to be a substrate for several members of this family of enzymes.
Although the reasons for this discrepancy remain unclear, it may be that not all cas-pases are inhibited by potassium. If this is the case, monitoring cleavage of a caspase substrate in cell lysates presumably containing many caspases, as opposed to the cleavage of caspase substrates by individual recombinant caspases (present study), would be predicted to generate different results. Nonetheless, the present data extend this earlier work by showing that in addition to procaspase processing, potassium directly inhibits active cas-pase-1 and caspase-3.