PROPERTIES OF THE REVERSIBLE, K-COMPETITIVE INHIBITOR OF THE GASTRIC (H+()K+)-ATPASE, SK-AND-F-97574 .1. IN-VITRO ACTIVITY/

SK&F 97574 yl-4(2-methylamino)-8-(2-hydroxyethoxy)quinoline), is a pot ent inhibitor of the (H+/K+)-ATPase in membrane vesicles isolated from porcine gastric mucosa. It inhibits (H+/K+)-ATPase activity in lyophi lised vesicles in a kinetically competitive manner with respect to the activating cation, K+, with an inhibition constant (K-i) of 0.46 +/- 0.003 mu M Inhibition of (H+/K+)-ATPase activity is freely reversible. Binding of SK&F 97574 was shown to be mutually exclusive and the prev iously reported reversible (H+/K+)ATPase inhibitors, SCH 28080 and MDP Q, Therefore, despite its structural dissimilarity, SK&F 97574 appears to bind to the same lumenal region of the (H+/K+)-ATPase identified a s the binding site for these compounds. SK&F 97574 is a weak base (pK( a) = 6.86), and would therefore be expected to accumulate in the acidi c compartment at the lumenal face of the parietal cell. In intact gast ric vesicles (which have the lumenal face of the ATPase on the interio r), SK&F 97574 inhibited ATP-dependent H+-transport with a similar pot ency to ATPase activity. SK&F 97574 is therefore relatively membrane p ermeable, and would be predicted to gain access readily to its site of action in vivo. The effect of pH on inhibition of H+/K+-ATPase activi ty by SK&F 97574 is consistent with its being active only in its proto nated form. The selectivity of SK&F 97574 for the gastric (H+/K+)-ATPa se was tested by examining its ability to inhibit a closely related p- class pump, the (Na+/K+)-ATPase from dog kidney. SK&F 97574 was found to have a 60-fold greater sensitivity for the former enzyme. The (Na+/ K+)-ATPase was not inhibited in a K+-competitive manner by SK&F 97574, indicating an entirely different, probably nonspecific, mechanism.