Effects of PP2A/PP1 inhibitors and of recombinant ENSA/ARPP19 proteins on Ser/Thr-protein phosphatase activities of platelet lysates analysed by a colorimetric peptide assay. (a) PP2A activity in lysates of human platelets after 0 or 60 min under basal conditions or in the presence of OA, fostriecin or tautomycetin (different concentrations). 0.1 nM and 1 nM of OA as well as 100 nM and 3 µM fostriecin completely inhibited PP2A activity in platelet lysates. 0.1 nM/1 nM tautomycetin had little effect in this assay, whereas 50 nM and 100 nM of the compound partially inhibited this PP2A activity. Data are expressed as percentage of free phosphate concentration compared to 100% basal free phosphate concentration after 60 min. Data are shown as mean of technical triplicates. (b) PP2A activity in lysates of human platelets after 0 and 60 min under basal conditions or in the presence of OA, recombinant HisENSA, HisENSA pS67, recombinant GST-ARPP19 and GST-ARPP19 pS62, respectively. Data are expressed as percentage of free phosphate concentration compared to 100% basal free phosphate concentration after 60 min, mean ± SD of three independent experiments. ** p < 0.01; *** p < 0.001; n.s.: not significant.