H + -pyrophosphatase (H + -PPase), which pumps H + across membranes coupled with PP i hydrolysis, is found in most plants, and some parasitic protists, eubacteria and archaebacteria. We assayed a number of extracts derived from 145 marine invertebrates as to their inhibitory effect on plant vacuolar H + -PPase. Acylspermidine derivatives [RCONH(CH 2 ) 3 N(CH 3 )(CH 2 ) 4 N(CH 3 ) 2 ] from a soft coral (Sinularia sp.) inhibited the PPi-hydrolysis activity of purified H + -PPase and the PP i

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... ndent H + pump activity (half inhibition concentration, 1 mM) of vacuolar membranes of mung bean. The apparent K i was determined to be 0.9 mM. Acylspermidines did not affect the activity of vacuolar H + -ATPase, plasma membrane H + -ATPase, mitochondrial ATPase or cytosolic PPase. Acylspermidines inhibited the acidification of vacuoles in protoplasts, as found on monitoring by the acridine orange fluorescent method. These results indicate that acylspermidine derivatives represent new inhibitors of H + -PPase with relatively high specificity.