On the basis of these observations different polyamines: diamine
putrescine, diamine cadaverine, triamine spermidine and the tetramine spermine, were tested. The IC50 values of these molecules are shown in Fig. 4A. Although spermine and spermidine were shown to support the activity of hammerhead [25–27] and hairpin [28,29] ribozymes in the presence or even in the absence of the required divalent cations, it appears that, unexpectedly, spermine strongly inhibits the CChMVd-HHR self-cleavage activity. The inhibitory effect of these different polyamines increases with the number of amino groups and the length of these molecules.
To obtain additional information about the influence of spermine on the activity of CChMVd-HHR, competition experiments between spermine and Mg2+ were performed. The Dixon plot obtained(Fig. 4B) yields a Ki of 17 ± 5 μMfor this inhibitor and the profile of the Dixon plot shows a non-competitive inhibition, indicating that spermine does not bind to the ribozyme on the Mg2+ binding site(s).