In vitro activity: Bafilomycin A1 is a toxic macrolide antibiotic derived from Streptomyces griseus. Bafilomycin A1 inhibits the short circuit current induced by the outer mantle epithelium (OME). The IC50 and maximum inhibition dose of Bafilomycin A1 are 0.17 μM and 0.5 μM, respectively. In addition, Bilomycin A1 inhibits the acid influx with an IC50 value of 0.4 nM. Bafilomycin A1 inhibits the acidification dose-dependently resulting in a lower quenching, and thus a higher fluorescence. Bafilomycin A1 prevents the vacuolization of Hela cells induced by H. pylori, with an inhibitory concentration giving 50% of maximal (ID50) of 4 nM. Bafilomycin A1 is also very efficient in restoring vacuolated cells to a normal appearance. Bafilomycin A1 also affects the transport of endocytosed material from early to late endocytic compartments. Bafilomycin not only dissipates the low endosomal pH but also blocks transport from early to late endosomes in HeLa cells.

Kinase Assay: The ATPase enzyme assay medium contains 6 mM MgSO4, 50 mM HEPES (pH 7.4), 200 mM Na2SO3 (V-ATPase activator), 0.5 mM sodium ortho-vanadate (P-ATPase inhibitor), 0.5 mM sodium azide (F-ATPase inhibitor) and 3 mM Na2ATP. This medium (1.0 mL), with or without the addition of the V-type ATPase inhibitor bafilomycin A1, is incubated with the filtered homogenate (0.1 mL) for 60 minutes at 23–25 °C. The reaction is stopped by the addition of 1 mL of TCA 3%. Spectrometric blanks are prepared as for the enzyme assay with the exception that the tissue sample is added after the acid. Phosphate analysis is accomplished by adding 2 mL of 1-butanol and 0.2 mL molybdate solution (5 g ammonium molybdate, 22 mL H2SO4 to 100 mL). After vortexing for 15 seconds the solution is neutralised with 0.5 mL citrate solution (100 g/500 mL, pH 7.0) and again vortexed for 15 seconds. The solution is then centrifuged (2000 × g; 3 minutes) to separate the butanol phase and the absorbance of this phase is read at 400 nm. Standards of orthophosphate are prepared (0.1 μM–2.0 μM) and treated in the same way as the enzyme activity assays. Enzyme activity is expressed in μmol of orthophosphate liberated per hour and per milligram of protein. V-ATPase activity is considered to be the difference between the total ATPase activity measured in the presence of Na2SO3, sodium orthovanadate and sodium azide and the ATPase activity measured in the presence of these reagents and of the specific V-ATPases inhibitor Bafilomycin A1.

Cell Assay: H. pylori bacteria extract is treated with inhibitors, before addition to HeLa cells, as follows: DCCD 10 mM for 1 hour at 30 oC; NBD-CI 100 μM for 1 hour at 30 oC and the reaction is blocked with glycine 10 mM final concentration; NEM 275 μM for 1 hour at 30 oC and the reaction is blocked by addition of β-mercaptoethanol 275 mM; Mg-ATP 14 μM for 1 hour at 0 oC; 100 μM KNO3 and 14 μM Mg-ATP for 1 hour at 30 μM; NaCO3 100 μM, pH 11 for 1 hour at 0 oC. The bacterial extract is then added to cell with a 40-fold dilution at a final concentration of 0.65 mg/mL. Controls are HeLa cells incubated with untreated bacterial extracts and cells treated with inhibitor Bafilomycin A1 under the same conditions as bacterial extracts, at the same concentrations or after a 40-fold dilution. The vacuotating activity of the bacterial extracts is assayed.

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