Effect of cyclic nucleotide analogs on intrachain site I of protein kinase isozymes

The effects of numerous cAMP analogs present in the [3H]cAMP binding reaction on subsequent dissociation of [3H]cAMP from the regulatory subunit of cAMP-dependent protein kinase I and II were analyzed. Certain analogs with modification at either C-8 or C-2 showed relative selectivity for one (site 1) of two intrachain cAMP binding sites of both isozymes. Modification at C-6 caused selectivity for the second site (site 2). The combination of a site-1-directed and site-2-directed analog inhibited [3H]cAMP binding much more than did either analog alone. In general, there was a correlation between the site 1 selectivity and the ability of the analog to stimulate the binding of [3H]cIMP, which selects site 2. The site-1-directed analogs stimulated the initial rate of [3H]cIMP binding. The stimulatory effect was enhanced in the presence of a polycationic protein such as histone and was inhibited by high ionic strength. The type I and II isozymes exhibited large differences in analog specificity for this effect. For type I, of the analogs tested the most efficacious for stimulating [3H]cIMP binding were those containing a nitrogen atom attached to C-8, 8-aminobutylamino-cAMP being the most effective. Type II responded best to analogs containing a sulfur atom attached to C-8, 8-SH-cAMP being the most effective of those tested. The stimulatory effect was accentuated in the presence of MgATP when using type I, but this nucleotide had no effect when using type II. It is proposed that in intact tissues cAMP binding to site 1 of either isozyme stimulates the binding to site 2.