Inhibition of eel enzymatic activities by cadmium

The aim of the present work was to study the in vitro effect of cadmium on enzymes, such as intestinal and branchial carbonic anhydrase (CA) and Na(+)-K(+)-ATPase which play a key role in salt- and osmoregulation and acid-base balance in the teleost fish, Anguilla anguilla. Carbonic anhydrase activities in gill and intestinal homogenates were significantly inhibited by CdCl(2), the gill CA being more sensitive to the heavy metal (IC(50) for the branchial CA=9.97+/-1.03x10(-6) M, IC(50) for the intestinal CA=3.64+/-1.03x10(-5) M, P<0.01). With regards to the intestinal CA activity, it has been shown in a previous study (Maffia et al., 1996) that two isoforms exist, a cytosolic and a brush-border membrane bound. These two isoforms show a different sensitivity to cadmium, with the membrane-bound enzyme less sensitive with respect to the cytosolic one, since it showed still an incomplete inhibition at the highest cadmium concentration tested. The inhibition of all the CA activity tested revealed a time-dependence since it required at least 10 min (1 h for the membrane-bound isoform) preincubation with the heavy metal to appear. Na(+)-K(+)-ATPase enzymatic activities, measured in intestinal and branchial homogenates, were inhibited by cadmium in a dose-dependent manner, with the branchial activity being more sensitive to the action of the heavy metal than the intestinal one (IC(50) for the branchial enzyme=1.38+/-0.09x10(-7) M, IC(50) for the intestinal enzyme=2.86+/-0.02x10(-7) M, P<0.01). The most of inhibition of the enzyme appeared without any preincubation with the heavy metal. Mg(2+)-ATPase activity was not significantly altered by the in vitro cadmium exposure either in the gills or in the intestine. These findings observed in vitro could be useful in the understanding of the toxic effects that cadmium elicits on aquatic organisms in vivo. In fact, the impairment of the activity of enzymes which carry out key physiological roles could cause alterations of the physiology of the whole organism.