The literature of the past four decades contains several scattered reports that demonstrate the existence of two different 3’,5’-cCMP degrading enzymes with distinct substrate affinity and specificity. The so-called “cCMP-specific” PDE was isolated from disrupted L-1210 leukemia cells  and from rat liver [2, 3], shows an extremely high 3’,5’-cCMP KM value of up to 9 mM and seems to be activated by iron . This enzyme prefers 3’,5’-cCMP over other cyclic nucleotides. The other enzyme is called “multifunctional cCMP PDE” and was isolated from pig liver [3–5], rat liver [6, 7] and human liver . This enzyme shows a 3’,5’-cCMP KM value in the range of 100-300 µM and is additionally degrading 2’,3’-cCMP as well as the 2’,3’- and 3’,5’-isomers of cUMP, cAMP, cGMP and cIMP. The multifunctional cCMP PDE is inhibited by inorganic phosphate and AMP. Both enzymes are calmodulin-insensitive, IBMX-resistant and show a low molecular weight of <35 kDa, which is uncommon for the “classic” phosphodiesterases. However, the exact amino acid sequence and identity of these proteins remain elusive.
Our project aims at a clear and unequivocal determination of amino acid sequence and identity of these elusive cCMP-degrading enzymes by analyzing and purifying the cCMP degrading activity of organ extracts and biological fluids (e.g. serum). Moreover, we re-investigate well-known “classic” PDEs in enzymatic assays to identify a potential 3’,5’-cCMP degrading activity.