FIG. 4.

Caspase-3 activity is increased in cells infected with MCF13 MLV. Cytosolic extracts were prepared from MCF13 MLV-infected (■), 4070A amphotropic MLV-infected (⧫), or mock-infected (○) cells on different days p.i. Caspase-3 activity was measured by using the EnzChek kit (Molecular Probes), which uses the Z-DEVD-AMC peptide as a substrate. Fluoromethylcoumarin fluorescence was detected with a SpectraMAX Gemini fluorometer (Molecular Devices). Units of caspase-3 activity are expressed as picomoles of substrate hydrolyzed per microgram of protein per half hour.