Identification of the novel rat liver IBMX-insensitive phosphodiesterase as a non-specific phosphodiesterase capable of hydrolysing cCMP

Inactivation of Non-canonical Cyclic Nucleotides: Hydrolysis and Transport

This chapter addresses cNMP hydrolysis by phosphodiesterases (PDEs) and export by multidrug resistance associated proteins (MRPs). Both mechanisms are well-established for the canonical cNMPs, cAMP, and cGMP. Increasing evidence shows that non-canonical cNMPs (specifically cCMP, cUMP) are also PDE and MRP substrates. Hydrolysis of cUMP is achieved by PDE 3A, 3B, and 9A, which possibly explains the cUMP-degrading activities previously reported for heart, adipose tissue, and brain. Regarding cCMP, the only known "conventional" (class I) PDE that hydrolyzes cCMP is PDE7A. Older reports describe cCMP-degrading PDE-like activities in mammalian tissues, bacteria, and plants, but the molecular identity of these enzymes is not clear. High K _M and V _max values, insensitivity to common inhibitors, and unusually broad substrate specificities indicate that these activities probably do not represent class I PDEs. Moreover, the older results have to be interpreted with caution, since the historical analytical methods were not as reliable as modern highly sensitive and specific techniques like HPLC-MS/MS. Besides PDEs, the transporters MRP4 and 5 are of major importance for cAMP and cGMP disposal. Additionally, both MRPs also export cUMP, while cCMP is only exported by MRP5. Much less data are available for the non-canonical cNMPs, cIMP, cXMP, and cTMP. None of these cNMPs has been examined as MRP substrate. It was shown, however, that they are hydrolyzed by several conventional class I PDEs. Finally, this chapter reveals that there are still large gaps in our knowledge about PDE and MRP activities for canonical and non-canonical cNMPs. Future research should perform a comprehensive characterization of the known PDEs and MRPs with the physiologically most important cNMP substrates.

From canonical to non-canonical cyclic nucleotides as second messengers: pharmacological implications

This review summarizes our knowledge on the non-canonical cyclic nucleotides cCMP, cUMP, cIMP, cXMP and cTMP. We place the field into a historic context and discuss unresolved questions and future directions of research. We discuss the implications of non-canonical cyclic nucleotides for experimental and clinical pharmacology, focusing on bacterial infections, cardiovascular and neuropsychiatric disorders and reproduction medicine. The canonical cyclic purine nucleotides cAMP and cGMP fulfill the criteria of second messengers. (i) cAMP and cGMP are synthesized by specific generators, i.e. adenylyl and guanylyl cyclases, respectively. (ii) cAMP and cGMP activate specific effector proteins, e.g. protein kinases. (iii) cAMP and cGMP exert specific biological effects. (iv) The biological effects of cAMP and cGMP are terminated by phosphodiesterases and export. The effects of cAMP and cGMP are mimicked by (v) membrane-permeable cyclic nucleotide analogs and (vi) bacterial toxins. For decades, the existence and relevance of cCMP and cUMP have been controversial. Modern mass-spectrometric methods have unequivocally demonstrated the existence of cCMP and cUMP in mammalian cells. For both, cCMP and cUMP, the criteria for second messenger molecules are now fulfilled as well. There are specific patterns by which nucleotidyl cyclases generate cNMPs and how they are degraded and exported, resulting in unique cNMP signatures in biological systems. cNMP signaling systems, specifically at the level of soluble guanylyl cyclase, soluble adenylyl cyclase and ExoY from Pseudomonas aeruginosa are more promiscuous than previously appreciated. cUMP and cCMP are evolutionary new molecules, probably reflecting an adaption to signaling requirements in higher organisms.

PDE7A1 hydrolyzes cCMP

The degradation and biological role of the cyclic pyrimidine nucleotide cCMP is largely elusive. We investigated nucleoside 3',5'-cyclic monophosphate (cNMP) specificity of six different recombinant phosphodiesterases (PDEs) by using a highly-sensitive HPLC-MS/MS detection method. PDE7A1 was the only enzyme that hydrolyzed significant amounts of cCMP. Enzyme kinetic studies using purified GST-tagged truncated PDE7A1 revealed a cCMP KM value of 135 ± 19 μM. The Vmax for cCMP hydrolysis reached 745 ± 27 nmol/(minmg), which is about 6-fold higher than the corresponding velocity for adenosine 3',5'-cyclic monophosphate (cAMP) degradation. In summary, PDE7A is a high-speed and low-affinity PDE for cCMP.

Conversion of 1-[((S)-2-hydroxy-2-oxo-1,4,2-dioxaphosphorinan-5-yl)methyl]cytosine to cidofovir by an intracellular cyclic CMP phosphodiesterase

Cidofovir (HPMPC) [1-[(S)-3-hydroxy-2-(phosphonomethoxy)propyl]-cytosine] is an acyclic nucleotide analog with potent and selective activity against herpesviruses. The prodrug, cyclic HPMPC (cHPMPC) [1-[((S)-2-hydroxy-2-oxo-1,4,2-dioxaphosphorinan-5-yl) methyl]cytosine], has antiviral activity similar to that of the parent compound but exhibits reduced toxicity in animal models. cHPMPC is converted to cidofovir by a cellular cyclic CMP phosphodiesterase (EC 3.1.4.37) which hydrolyzes a variety of substrates, including adenosine 3',5'-cyclic monophosphate (cAMP) and cytidine 3',5'-cyclic monophosphate (cCMP). The K(m) and Vmax values for hydrolysis of cHPMPC by cCMP phosphodiesterase purified from human liver are 250 microM and 0.66 nmol.min-1.unit-1, respectively. These values are similar to the K(m) and Vmax values for cAMP (23 microM and 1.16 nmol.min-1.unit-1, respectively) and cCMP (75 microM and 2.32 nmol.min-1.unit of enzyme-1, respectively). The catalytic efficiency (Vmax/K(m) ratio) of this enzyme for the cHPMPC substrate is only 10- to 20-fold lower than those for the natural cyclic nucleotides, indicating that cHPMPC is a viable intracellular substrate for the human enzyme. Kinetic analysis indicates that cHPMPC, cAMP, and cCMP are competitive with respect to each other and that they are hydrolyzed by the same enzyme. cHPMPC is hydrolyzed to cidofovir in all primary human cell systems tested, including those derived from target organs that might be infected in patients with human cytomegalovirus (HCMV) disease. Importantly, hydrolysis of cHPMPC is not diminished in cells infected with HCMV.