Abstract: | When microsomal membranes from maize (Zea mays L. cv. Clipper)coleoptiles were separated by isopyc-nic centrifugation on acontinuous 1045% sucrose gradient, bafilomycin A1-inhibitedATPase activity co-localized with the activities of the tonoplastmarker-enzymes, nitrate-Inhibited ATPase and K+-dependent pyrophosphatase.Thus, bafilomycin A1 is a specific inhibitor of the vacuolarH+-ATPase of maize coleoptiles. Inhibition of the vacuolar H+-ATPaseby bafilomycin A1 was strictly dependent upon the concentrationof the enzyme present in the assay medium, suggesting a stoichiometricassociation between bafilomycin A1 and the vacuolar H+-ATPase.In tonoplast-enriched preparations, half-maximal inhibitionwas obtained at 43 pmol bafilomycin A1 mg1 protein. BafilomycinA1 inhibited the vacuolar H+-ATPase in a simple non-competitivemanner: increasing bafilomycin A1 concentrations reduced theVmax, of the H+ -ATPase, but had no effect on its Km towardsATP. Key words: Bafilomycin A1, coleoptile, H+-ATPase (vacuolar), maize, Zea mays L |