PROPERTIES OF OUABAIN-RESISTANT NA+/K+-TRANSPORTING ATPASE FROM THE EXCRETORY SYSTEM OF POEKILOCERUS-BUFONIUS

The properties of Na+/Ka+ -transporting ATPase in microsomal fractions from the excretory system (Malpighian tubules plus hind-gut) of Poekilocerus bufonius were studied. The optimal conditions required for optimal activity of Na+/K+ -transporting ATPase were as follows: (1) Maximal activation of Na+/K+ -transporting ATPase occurred at an ATP:Mg2+ ratio of 1:1.5. The apparent K(m) value was 0.84 mM and the V(max) was 166 nmol Pi lib. mg protein-1 min-1. (2) Maximal activation of Na+/K+ -transporting ATPase occurred at 80 mM Na+ and 20 mM K+. The apparent K(m) values were 7.4 mM Na+ and 3.6 mM K+. (3) The optimal pH of Na+/K+ -transporting ATPase was 7.5. (4) The enzyme is rather insensitive to the cardiac glycoside ouabain (I50 = 2 x 10(-4)M). The sensitivity of Na+/K+ -transporting ATPase of P. bufonius to Na+, K+ and ouabain is lower than that of S. gregaria. However, the low sensitivity of the enzyme to ouabain inhibition observed in this insect would possibly indicate that the presence of this enzyme is an important factor in the tolerance of plant cardiac glycosides in P. bufonius.