Prednisolone-3,20-bisguanylhydrazone: the mode of interaction with rat brain sodium and potassium-activated adenosine triphosphatase.

The mechanism of interaction between prednisolone-3,20-bisguanylhydrazone (PBGH) and Na+,K+-ATPase (ATP phosphohydrolase, EC 3.6.1.3) was studied using partially purified rat brain enzyme preparations. PBGH inhibited Na+,K+-ATPase rapidly and reversibly. The enzyme-inhibiting action of PBGH was competitively antagonized by potassium. PBGH inhibited Na+,Mg2+ and ATP-supported binding of (3H)-ouabain to the enzyme. When PBGH was added to the incubation mixture at the time when the (3H)-ouabain binding was close to its equilibrium state, the concentration of (3H)-ouabain complex was rapidly reduced and shifted to a lower equilibrium state. A double reciprocal plot analysis of the (3H)-ouabain binding data indicates that the inhibition of ouabain binding by PBGH is apparently competitive. Binding of (3H)-ouabain in the presence of Tris-phosphate and Mg2+ was also inhibited by unlabeled PBGH. Thus, it appears that the binding of PBGH precludes the binding of ouabain to Na+,K+-ATPase.