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The threat of a dirty bomb which could cause internal contamination has been of major concern for the past decades. Because of their high chemical toxicity and their presence in the nuclear fuel cycle, uranium and neptunium are two actinides of high interest. Calmodulin (CaM) which is a ubiquitous protein present in all eukaryotic cells and is involved in calcium-dependent signaling pathways has a known affinity for uranyl and neptunyl ions. The impact of the complexation of these actinides on the physiological response of the protein remains, however, largely unknown. An isothermal titration calorimetry (ITC) was developed to monitor in vitro the enzymatic activity of the phosphodiesterase enzyme which is known to be activated by CaM and calcium. This approach showed that addition of actinyl ions (AnO 2 n+ ), uranyl (UO 2 2+ ) and neptunyl (NpO 2 + ), resulted in a decrease of the enzymatic activity, due to the formation of CaM-actinide complexes, which inhibit the enzyme and alter its interaction with the substrate by direct interaction. Results from dynamic light scattering rationalized this result by showing that the CaM-actinyl complexes adopted a specific conformation different from that of the CaM-Ca 2+ complex. The effect of actinides could be reversed using a hydroxypyridonate actinide decorporation agent (5-LIO(Me-3,2-HOPO)) in the experimental medium demonstrating its capacity to efficiently bind the actinides and restore the calcium-dependent enzyme activation., (Copyright © 2017 Elsevier Inc. All rights reserved.)