Properties of silkworm Na+/K+-ATPase

Abstract

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 Mg(2+) concentration, pH and sensitivities to Ca(2+) 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 α-subunit of silkworm Na(+)/K(+)-ATPase.