SKAGGS, H.S.*; SNYDER, E.; HENRY, R.P.: Heavy metal effects on carbonic anhydrase in Callinectes sapidus and Carcinus maenas

Branchial carbonic anhydrase (CA) in euryhaline crustaceans is known to function in active ion uptake and osmoregulation in low salinity. This function could be compromised by heavy metals in the aquatic environment. Cytoplasmic CA from gills of Callinectes sapidus and Carcinus maenas was titrated against increasing concentrations of Ag, Cu, Cd, and Zn in vitro to determine the inhibitory effects on the enzyme. All metals were strong inhibitors of CA from C. sapidus, with Ki values of 0.39, 0.40, 0.65 and 3.88 nM, respectively. Concentrations needed to affect 100% inhibition were also low: 3.3, 17, 17, and 166 nM, for Ag, Cu, Cd, and Zn, respectively. In contrast, cytoplasmic CA from gills of C. maenas was much less sensitive to heavy metal inhibition. Concentrations of Cu, Cd, and Zn needed to achieve 100% inhibition in vitro were 4.2, 5.0, and 12.5 M, respectively, approximately 100 to 1000 fold higher than those for C. sapidus. This suggests the presence of a metal-resistant isoform in the gills of C. maenas that may help protect the ion transport mechanism against heavy metal toxicity. When individuals of C. maenas acclimated to 35 ppt salinity and exposed to three concentrations of Cd (0.1, 1.0, and 10 M) were transferred to low salinity (10 ppt), mortality was no different than in control, untreated crabs. Furthermore, hemolymph osmotic concentrations in both control and treated crabs stabilized by 12 hr post-transfer at between 650 and 700 mOsm.