A radioisotope assay was used to study the inhibition of protein phosphatase 1 and 2A activity by microcystin-LR in crude extracts of three zooplankton species. The enzyme activities of extracts from Daphnia pulex were 3-5 times lower than the activities of extracts from Daphnia pulicaria and Diaptomus birgei. However, the protein phosphatase activity of D. pulex was less sensitive to inhibition by the toxin (IC 50 of 0.2 nM) than the activities of D. pulicaria (IC 50 of 0.02 nM) and Diaptomus birgei (IC 50 of 0.01 nM). These results are compared with previous studies that tested the effects of purified toxins, cell extracts and toxic cyanobacteria on live zooplankton of the same species. Our results verify that microcystin-LR is a potent inhibitor of the protein phosphatase activities of these zooplankton. The impact of microcystin on living zooplankton may also depend on feeding behavior, absorption of toxin in the gut, and abilities to synthesize additional enzyme.
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DeMott et al. (1995) studied this question.