Comparison of enzymatic characterization and gene organization of cyclic nucleotide phosphodiesterase 8 family in humans.

Full-length cDNAs of human cyclic nucleotide phosphodiesterase 8B (PDE8B) were isolated. Enzymatic characteristics of a dominant variant encoding a protein of 885 residues (PDE8B1) were compared with those of PDE8A1. The recombinant PDE8A1 and PDE8B1 proteins of an entire form were produced in both cytosolic and membrane fractions of the transfected COS cells. The human PDE8B1 was a high-affinity cAMP-PDE with K(m) value of 101+/-12 nM for cAMP, which is greater than that of PDE8A1 (40+/-1 nM). Relative V(max) value of PDE8A1 was 57+/-8% compared with that of PDE8B1 (100+/-12%). Although PDE8A1 was moderately inhibited by dipyridamole with IC(50) value of 8+/-2 microM, the compound antagonized the PDE8B1 activity at three-fold higher concentration (IC(50)=23+/-2 microM). The human PDE8B gene was composed of 22 exons, spanning over 217 kb. Although overall sequence identity between PDE8A1 and PDE8B1 was 68%, positions of junctions of each exon between the PDE8A1 and PDE8B1 sequences were well matched, indicating evolutionary relatedness of both genes.