| Abstract: | The two exocyclic oxygen atoms at phosphorus of cAMP have been replaced by a sulfur atom or by a dimethylamino group. These substitutions introduce chirality at the phosphorus atom; therefore, two diastereoisomers are known for each derivative: (SP)-cAMPS, (RP)-cAMPS, (SP)-cAMPN(CH3)2, and RP-cAMPN(CH3)2. We have investigated the agonistic and antagonistic activities of these compounds in four cAMP-dependent reactions: activation of the cellular slime mold Dictyostelium discoideum via its cell surface cAMP receptor, and phosphorylation by cAMP-dependent protein kinases type I, type II (both mammalian enzymes), and type D (derived from D. discoideum). The results show that 1) the compounds (SP)-cAMPS and (SP)-cAMPN(CH3)2 are (mostly full) agonists for the four proteins. Half-maximal activation is at micromolar concentrations (0.8-7 microM). 2) (RP)-cAMPS is a full antagonist for the cell surface receptor and protein kinases type I and II, with apparent inhibition constants between 0.8 and 8 microM. This compound is a partial agonist for protein kinase type D, where it induces maximally 50% activation of the enzyme if compared with cAMP. 3) (RP)-cAMPN(CH3)2 is a full antagonist for the cell surface receptor, and for protein kinase type II. This compound is a partial agonist for protein kinase type I (at least 50% activation if compared with cAMP), and inactive for protein kinase type D. This derivative is at least 25-fold less active as an antagonist than (RP)-cAMPS. 4) The activity of mixtures of different concentrations of the antagonist (RP)-cAMPS with different concentrations of cAMP reveals that the compound is a competitive antagonist of cAMP at micromolar concentrations. |
