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 ba lance in the teleost fish, Anguilla anguilla. Carbonic anhydrase activities in gill and intestinal homogenates were significantly inhibited by CdCl2, the gill CA bring more sensitive to the heavy metal (IC50 for the branchial CA = 9.97 +/- 1.03 x 10(-6) M, IC50 for the intestinal CA = 3.64 +/- 1.03 x 10(-5) M, P<0.01). With regards to the intestinal CA activity, it has bee n 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 di fferent sensitivity to cadmium, with the membrane-bound enzyme less sensiti ve with respect to the cytosolic one, since it showed still an incomplete i nhibition at the highest cadmium concentration tested. The inhibition of al l the CA activity tested revealed a time-dependence since it required at le ast 10 min (1 h for the membrane-bound isoform) preincubation with the heav y metal to appear. Na+-K+-ATPase enzymatic activities, measured in intestin al 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 (IC50 for the branchial enzyme = 1. 38 +/- 0.09 x 10(-7) M, IC50 for the intestinal enzyme = 2.86 +/- 0.02 x 10 (-7) M. P < 0.01). The most of inhibition of the enzyme appeared without an y preincubation with the heavy metal. Mg2+-ATPase activity was not signific antly altered by the in vitro cadmium exposure either in the gills or in th e intestine. These findings observed in vitro could be useful in the unders tanding of the toxic effects that cadmium elicits on aquatic organisms in v ivo. 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. (C) 2000 Elsevier Science B.V. All rights reserved.