Inhibition of Na+/K+-ATPase and of active ion-transport functions in the gills of the shore crab Carcinus maenas induced by cadmium |
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Authors: | U Postel G Petrausch S Riestenpatt D Weihrauch J Malykh W Becker D Siebers |
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Institution: | (1) Biologische Anstalt Helgoland, Notkestr. 31, D-22607 Hamburg, Germany, DE;(2) Shemyakin Institute of Bioorganic Chemistry, ul. Miklukho – Maklaya 16/10 GSP-7 Moscow, 117871, Russia, RU;(3) Zoologisches Institut und Museum, Martin-Luther-King-Platz 3, D-20146 Hamburg, Germany, DE |
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Abstract: | Inhibition of Na+/K+-ATPase from gill plasma membranes of the shore crab Carcinus maenas by cadmium was investigated and compared with inhibitory effects by known antagonists (ouabain and Ca2+). For comparative considerations the Cd2+-inhibition of the enzyme from dog kidney was also tested. Na+/K+-ATPase from dog kidney and from crab gill differed greatly in sensitivity against ouabain. The inhibition constant K
i of the dog enzyme amounted to 9.1 × 10−7 mol l−1, i.e. more than 300-fold smaller than the K
i of 2.9 × 10−4 mol l−1 determined for the crab enzyme. Ca2+ inhibited the activity of Na+/K+-ATPase from crab gill plasma membranes with a K
i of 4.3 × 10−4 mol l−1. The Na+/K+-ATPase from crab gill was inhibited by Cd2+ with a K
i of 9.1 × 10−5 mol l−1. Cd2+ inhibited the Na+/K+-ATPase from dog kidney with a K
i (6.4 × 10−5 mol l−1) comparable to that observed in the crab gill enzyme. Under experimental conditions Cd2+-inhibition of Na+/K+-ATPase was irreversible. Repeated washing, centrifugation and homogenization of the plasma membranes (four times) with Cd2+-free buffer did not restore any activity lost in the presence of 1 × 10−3 mol l−1 Cd2+. Since ouabain-insensitive (nonspecific) ATPases in the plasma membrane fraction of crab gills were inhibited by Cd2+ in the same way as Na+/K+-ATPase, the heavy metal is considered as an unspecific ATPase inhibitor. Comparing these results with literature data on
Cd2+-binding to electrophoretically separated proteins suggests that Na+/K+-ATPase is a Cd2+-binding enzyme. The results obtained on Na+/K+-ATPase were reflected by Cd2+-inhibition of the branchial ion-transport functions depending on this enzyme. The transepithelial short-circuit current of
isolated gill half lamellae, a direct measure of area-specific active ion uptake, and the transepithelial potential difference
of isolated, perfused whole gills, also indicative of active ion uptake, were inhibited by the heavy metal in a time- and
dose-dependent mode. Remarkably these inhibitions were also irreversible. These findings are ecologically and biomedically
significant: even when the actual environmental or tissue concentrations measured are low, biological microstructures such
as Na+/K+-ATPase may accumulate the heavy metal by tight binding over prolonged periods until the first inhibitory effects occur.
Received: 25 June 1997 / Accepted: 25 August 1997 |
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