Ac-LEHD-CHO is a peptide aldehyde inhibitor directed at caspase-9, the initiator caspase of the intrinsic apoptotic pathway. The LEHD recognition sequence is the same one used in the corresponding chromogenic and fluorogenic substrates, Ac-LEHD-pNA and Ac-LEHD-AFC, so substrate and inhibitor share a specificity profile. The C-terminal aldehyde is what converts a substrate into an inhibitor. It forms a reversible covalent hemithioacetal with the catalytic cysteine of the protease, which makes the inhibition tight but reversible, distinguishing this class from irreversible fluoromethyl ketone inhibitors such as Z-LEHD-FMK. Reversibility matters experimentally: activity can return on dilution or washout, which is useful where transient inhibition is wanted and a pitfall where sustained blockade is assumed. The more important caveat concerns selectivity. Caspase tetrapeptide preferences overlap substantially, and LEHD-based reagents are selective for caspase-9 only in relative terms; at higher concentrations they inhibit other caspases, particularly caspase-4 and caspase-5. Attributing an effect specifically to caspase-9 therefore requires a concentration titration and ideally an orthogonal approach rather than the inhibitor alone. Work using caspase-8 and caspase-9 inhibitors to improve viability in CHO cell cultures illustrates the practical application of this class. | 