Effects of (Na⁺+K⁺-ATPase-specific antibodies on enzymatic activity and monovalent cation transport

Antibodies have been obtained that specifically interact with the transport enzyme (Na⁺+K⁺-activated ATPase. The antigen used was purified (Na⁺+K⁺)-ATPase from canine renal medulla. Purified γ globulin from immunized animals, but not from control animals or preimmune serum, inhibited (Na⁺+K⁺)-ATPase from canine renal medulla with reduction of activity to 33±4 (sd) % in a concentration-dependent manner. Maximum inhibition occurred in less than 5 minutes at 37°C. The Mg⁺⁺-dependent, nonouabain inhibited component of activity (Mg⁺⁺-ATPase) was unaffected. Fab fragments obtained by papain cleavage of the γ globulin fraction had similar inhibitory activity and specificity. These antibodies also produced varying degrees of concentration-related inhibition of canine myocardial, calf brain, and human red cell ghost (Na⁺+K⁺)-ATPase, but not Mg⁺⁺-ATPase activity. Despite marked inhibition of (Na⁺+K⁺)-ATPase activity in these enzyme preparations from disrupted cells, experiments with canine renal slices, guinea pig atrial strips, and human red cells showed no specific effect of antibody on ouabain-inhibitable⁸⁶Rb⁺ uptake, indicating a lack of inhibition of active monovalent cation transport in the intact cell. When specific antibody had access to the inner surface of lysed, resealed human red cell ghosts, however, complete inhibition of (Na⁺+K⁺)-ATPase-mediated²²Na⁺ efflux was observed. Consistent with this finding was complete inhibition of (Na⁺+K⁺)-ATPase activity in inside-out, but not right-side-out, red cell membrane vesicles. These experiments demonstrate immunologic cross-reactivity among (Na⁺+K⁺)-ATPases from different organs and different species. In addition, they indicate that the antibody response resulting in enzyme and transport inhibition is directed against an antigenic determinant or determinants inaccessible to macromolecules at the outer cell surface.