EFFECTS OF MICROCYSTIN-LR ON THE PARTIAL REACTIONS OF THE NA-K+ PUMP OR THE GILL OF CARP (CYPRINUS-CARPIO LINNEO)()

Several partial reactions of the Na+-K+ pump enzyme were studied in a microsomal fraction derived from the gill of carp (Cyprinus car-pie Li nneo). We tested the effect of three toxins [(i) microcystin-LR, (ii) microcystin-LR-like toxin component isolated from Microcystis aerugino sa culture and (iii) okadaic acid] on the phosphorylation, ouabain bin ding and ATPase activity of the Na+-K+ pump. The K+-dependent hydrolys is of the Na+-dependent phosphorylation of Na+-K+ pump, as well the re lease of bound ouabain were inactivated in direct proportion to the am ount of each toxin treatment. These results indicate that these toxins not only block the hydrolysis of phosphorylated protein at serine and threonine residues, but also inhibit the aspartic dephosphorylation s tep of the sodium pump enzymes. This inactivation could disrupt the io n homeostasis of the internal medium by blocking the gill function. Th e blockage of gill activity could be the cause of the massive fish dea th during blooms of M. aeruginosa. (C) 1996 Elsevier Science Ltd.