Inhibition of Na+,K+-ATPase in Penaeus indicus postlarvae by lead

The plasma membrane/mitochondrial fractions of Penaeus indicus postlarvae contain Mg²⁺-dependent ATPase, Na⁺,K⁺-stimulated ATPase, Na⁺-stimulated ATPase and K⁺-stimulated ATPase. The Na⁺,K⁺-activated, Mg²⁺-dependent ATPase was investigated further in relation to different pH and temperature conditions, and at various concentrations of protein, ouabain, ATP and ions in the incubation medium. In vitro and in vivo effects of lead were studied on the enzyme activity. In vitro lead inhibited the enzyme activity in a concentration-dependent manner with an IC₅₀ value of 204.4 μM. In correlation with in vitro studies, in vivo investigations (both concentration and time dependent) of lead also indicated a gradual inhibition in enzyme activity. A maximum decrease of 85.3% was observed at LC₅₀ (7.2 ppm) of lead for concentration-dependent experiments. In time-dependent studies, the decrease was maximal (81.7%) at 30 days of sublethal exposure (1.44 ppm). In addition, the substrate- and ion-dependent kinetics of Na⁺,K⁺-ATPase was studied in relation to in vitro exposure of lead; these studies suggest a non-competitive type of inhibition.