Table 2.
Properties of six Dictyostelium PDEs
| Phosphodiesterase | PDE1 | PDE2 | PDE3 | PDE4* | PDE5 | PDE6 |
|---|---|---|---|---|---|---|
| Name of gene | psdA | regA | DdPDE3 | DdPDE4 | gbpA | gbpB |
| Localization | Cell surface | Cytosol | Cytosol | Cell surface | Cytosol | Cytosol |
| Class | II | I | I | I | II | II |
| cAMP/cGMP selectivity | 3 | >200 | ∼0.0015 | Unknown | <0.003 | 9 |
| cAMP hydrolysis | ||||||
| KM (μM) | 0.8 | 5 | >100 | Unknown | >500 | 200 |
| (150) | (1800) | |||||
| VMAX (pmol/min/mg) | 700 | 50 | – | Unknown | – | 5200 |
| KA (μM) | – | – | – | Unknown | >300 | 0.7 |
| AMAX | – | – | – | – | – | 1.47 |
| cGMP hydrolysis | ||||||
| KM (μM) | 1.8 | >1000 | 0.22 | –* | 5.2–20 | 800 |
| VMAX (pmol/min/mg) | 490 | – | 2 | –* | 390 | 2400 |
| KA (μM) | – | – | – | –* | 0.16 | 2.3 |
| AMAX | – | – | – | – | 2.40 | 1.86 |
| Intracellular cAMP degradation (pmol/min/mg) | ||||||
| 0.1 μM | – | 1 | – | –* | – | 2.6 |
| 1 μM | – | 8 | – | –* | – | 26 |
| 5 μM | – | 25 | – | –* | – | 127 |
| Intracellular cGMP degradation (pmol/min/mg) | ||||||
| 0.1 μM | – | – | 0.6 | –* | 3.3 | 0.3 |
| 1 μM | – | – | 1.6 | –* | 48 | 3.0 |
| 5 μM | – | – | 1.9 | –* | 163 | 15 |
PDE4
, the enzyme has not been characterized biochemically; sequence data suggest that the enzyme is cAMP specific and has a signal sequence and two transmembrane segments predicting the catalytic domain to be extracellular. The cAMP/cGMP selectivity refers to the calculated VMAX/KM for cAMP divided by the VMAX/KM for cGMP. The data for the KM in parentheses refer to the cAMP concentration inducing half-maximal inhibition of the hydrolysis of cGMP. The kinetic constants were derived from Van Haastert et al. (1983) for PDE1; from Shaulsky et al. (1998) and Thomason et al. (1998) for PDE2, and from Figure 4 to calculate the VMAX in vivo, assuming that cAMP hydrolysis in gbpA−/gbpB− is derived from RegA; for PDE6 from Kuwayama et al. (2001) for PDE3; from Figure 7 for PDE5; and from Figure 6 for PDE6. The rates of intracellular degradation were calculated using the obtained kinetic constants and are presented for three concentrations representing basal levels (0.1 μM), maximal levels in wild-type cells (1 μM), and maximal levels in some deletion mutants (5 μM).