Salinity-sensitive alkaline phosphatase activity in gills of the blue crab,Callinectes sapidus Rathbun

A levamisole-sensitive (K_i = 0.72 mM) alkaline phosphatase (pH optimum 9.1) and a levamisole-insensitive alkaline phosphatase (pH optimum 7.1) are present in gills of the blue crab Callinectes sapidus. Both enzymes are distinct from ouabain-sensitive ATPase. Specific activity for either phosphatase is greatest in the acinar tissue, which lines the branchial vessels. Histochemical localization of the enzymes confirmed this distribution. Activity of levamisole-sensitive alkaline phosphatase is affected by acclimation salinity. V_max of the levamisole-sensitive alkaline phosphatase is greater in high-salinity crabs than in low-salinity crabs; apparent K_m is not significantly different. The levamisole-sensitive alkaline phosphatase associated with the acinar tissue lining the branchial vessels may modulate the osmoregulatory response in blue crabs.