Properties of silkworm Na+/K+-ATPase

The high Na+ and low K+ concentrations in mammalian blood are maintained by Na+/K+-ATPase. In contrast, the K+ concentration is higher than the Na+ concentration in the hemolymph of the silkworm Bombyx mori, a Lepidopterous insect. Although Na+/K+-ATPase, therefore, appears not to be in silkworm, we confirmed the presence of Na+/K+-ATPase in nerve tissues of silkworm but not in skeletal muscle or the dorsal vessel. The enzymatic properties of silkworm Na+/K+-ATPase were characterized in detail and compared with those of dog Na+/K+-ATPase. Silkworm Na+/K+-ATPase had a much lower affinity for K+ and a somewhat higher affinity for Na+ than dog Na+/K+-ATPase. The optimal temperature of silkworm Na+/K+-ATPase activity was lower than that of dog Na+/K+-ATPase. The optimal Mg2+ concentration, pH and sensitivities to Ca2+ and ouabain, a specific inhibitor of Na+/K+-ATPase, of the two ATPases were identical. These results indicate that the enzymatic properties of the silkworm Na+/K+-ATPase are suitable for its growth, despite the differences between dog and silkworm Na+/K+-ATPases. Antisera raised against dog Na+/K+-ATPase recognized only the alpha-subunit of silkworm Na+/K+-ATPase.