The enzymatic activities of PDE4D catalytic domain and
inhibition of isolated compounds were assayed in a reaction
mixture containing 50 mM Tris–HCl (pH 7.5), 10 mM MgCl2,
0.5 mM dithiothreitol, 10 μM inhibitor (dissolved in DMSO)
or equivalent volume of DMSO without inhibitor (for
negative control) and 20 000–30 000 cpm per assay substrate
3H-cAMP at room temperature for 15 min. Then the
reaction was terminated by addition of 0.2 M ZnSO4 and the
unreacted 3H-cAMP was precipitated out by 0.2 N Ba(OH)2
whereas remained in the supernatant. The radioactivity in
the supernatant was measured in 2.5 mL of Ultima Gold
liquid scintillation cocktails by a liquid scintillation counter.
Each testwas repeated at least for three times. In this bioassay,
a PDE4D inhibitor rolipram was used as the reference
compound.