Irreversible inhibition of calmodulin-sensitive cyclic nucleotide phosphodiesterase

Photolysis of solutions containing 4-azido-7-phenylpyrazolo-[1,5a]-1,3,5-triazine (APPT) and calmodulin-sensitive cyclic nucleotide phosphodiesterase resulted in reduction of both cyclic GMP and cyclic AMP hydrolytic activity. The inactivation was dependent upon both time of exposure to ultraviolet irradiation and the initial concentration of APPT. The photo-induced inactivation could be attenuated by the presence of cyclic GMP, 1-methyl-3-isobutylxanthine, and papaverine. alpha-Chymotrypsin treatment caused the enzyme to be fully active in the absence of calmodulin but this treatment did not alter the ability of APPT to inactivate the enzyme. Thus, inhibition of calmodulin-binding did not contribute to the photo-induced inactivation. These data indicate that APPT acts as a photoaffinity agent to covalently modify the APPT-binding site of calmodulin-sensitive phosphodiesterase.