Identification and stabilization of large molecular weight PDE-IVs from U937 cells

Cytosolic cyclic nucleotide phosphodiesterases (PDEs) from human (promonocytic) U937 cells were rapidly resolved by DEAE-Sepharose CL-6B anion exchange chromatography into two major peaks of cAMP-specific activity possessing average K_ms of 1.70 μM (Peak 1) and 1.65 μM (Peak 2). Both peaks were predominantly PDE-IV, but possessed molecular weights higher than those generally reported for partially purified PDE-IVs. Storage of Peak 2 for 24 h at 4°C resulted in a doubling of its V_max and an apparent decrease in its molecular weight. Activation of Peak 2 PDE-IV was prevented when the sodium acetate concentration in its buffer was reduced by dilution immediately following isolation. Although the relevance of this activation to cellular regulation of PDE-IV is undefined, the isolation and stabilization of PDE-IV in its large molecular weight form will be critical to future investigations of PDE-IV regulation.