INHIBITION OF IN-VITRO PROTEIN PHOSPHATASE-ACTIVITY IN 3 ZOOPLANKTON SPECIES BY MICROCYSTIN-LR, A TOXIN FROM CYANOBACTERIA

A radioisotope assay was used to study the inhibition of protein phosp hatase 1 and 2A activity by microcystin-LR in crude extracts of three zooplankton species. The enzyme activities of extracts from Daphnia pu lex were 3-5 times lower than the activities of extracts from Daphnia pulicaria and Diaptomus birgei. However, the protein phosphatase activ ity of D. pulex was less sensitive to inhibition by the toxin (IC50 of 0.2 nM) than the activities of D. pulicaria (IC50 of 0.02 nM) and Dia ptomus birgei (IC50 of 0.01 nM). These results are compared with previ ous studies that tested the effects of purified toxins, cell extracts and toxic cyanobacteria on live zooplankton of the same species. Our r esults verify that microcystin-LR is a potent inhibitor of the protein phosphatase activities of these zooplankton. The impact of microcysti n on living zooplankton may also depend on feeding behavior, absorptio n of toxin in the gut, and abilities to synthesize additional enzyme.