Molecular cloning and characterization of human PDE8B, a novel thyroid-specific isozyme of 3',5'-cyclic nucleotide phosphodiesterase.

We have identified a novel human isozyme of 3',5'-cyclic nucleotide phosphodiesterase (PDE), which we designated PDE8B. cDNA of 2844 bp encoding the C-terminal 659 amino acids of PDE8B was cloned following the identification of an expressed sequence tag (EST) obtained through a search of the EST database. The predicted protein sequences of PDE8B showed highest homology (65% identity, 83% similarity) to that of PDE8A. Northern blot analysis indicated that the mRNA encoding PDE8B is expressed specifically and abundantly in thyroid gland as a approximately 4.2 kb mRNA, in contrast to the wide expression of PDE8A mRNA in various tissues. The carboxyl-terminal 584 amino acids of PDE8B were expressed in E.coli as a fusion protein. The recombinant PDE8B exhibited cAMP PDE activity which was not inhibited by various PDE inhibitors including vinpocetine, milrinone, rolipram, and IBMX with the exception of dipyridamole which caused 50% inhibition at a concentration of 40 microM. cAMP hydrolytic activity was unaffected by cGMP and no cGMP PDE hydrolysis were detectable at concentrations up to 100 microM. These findings suggest that PDE8B is a new member of the PDE8 family.