Inhibition of cGMP‐dependent protein kinase by (Rp)‐guanosine 3',5'‐monophosphorothioates

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Abstract
The activation of the cGMP‐dependent protein kinase and cAMP‐dependent protein kinase by the diastereomers of guanosine 3',5'‐monophosphorothioate, (Sp)‐cGMPS and (Rp)‐cGMPS, and 8‐chloroguanosine 3',5'‐monophosphorothioate, (Sp)‐8‐Cl‐cGMPS and (Rp)‐8‐Cl‐cGMPS, was investigated using the peptide Kemptide as substrate. The (Sp)‐diastereomers, which have an axial exocyclic sulfur atom, bound to the cGMP‐dependent protein kinase and stimulated its phosphotransferase activity. In contrast, the (Rp)‐isomers, which have an equatorial exocyclic sulfur atom, bound to the enzyme without stimulation of its activity. (Rp)‐cGMPS and (Rp)‐8‐Cl‐cGMPS antagonized the activation of the cGMP‐dependent protein kinase with a K i of 20 μM and 1.5 μM, respectively. (Rp)‐cGMPS also antagonized the activation of cAMP‐dependent protein kinase with a K i of 20 μM. In contrast, (Rp)‐8‐Cl‐cGMPS was a weak inhibitor of the cAMP‐dependent protein kinase with a K i of 100 μM. (Rp)‐8‐Cl‐cGMPS appears to be a rather selective inhibitor of the cGMP‐dependent protein kinase and may be a useful tool for studying the role of cGMP in broken and intact cell systems.