The human epidermis holds an autocrine acetylcholine production and degradation including functioning membrane integrated and cytosolic butyrylcholinesterase (BuchE). Here we show that BuchE activities increase 9-fold in the presence of calcium (0.5x10(-3)M) via a specific EF-hand calcium binding site, whereas acetylcholinesterase (AchE) is not affected. (45)Calcium labelling and computer simulation confirmed the presence of one EF-hand binding site per subunit which is disrupted by H(2)O(2)-mediated oxidation. Moreover, we confirmed the faster hydrolysis by calcium-activated BuchE using the neurotoxic organophosphate O-ethyl-O-(4-nitrophenyl)-phenylphosphonothioate (EPN). Considering the large size of the human skin with 1.8m(2) surface area with its calcium gradient in the 10(-3)M range, our results implicate calcium-activated BuchE as a major protective mechanism against suicide inhibition of AchE by organophosphates in this non-neuronal tissue.