When mitochondrial ATPase, which has been modified on a single tyrosine residue by 4-chloro-7-nitrobenzofurazan, is incubated at pH 9.0, the 7-nitrobenzofurazan group undergoes an intramolecular transfer to a nitrogen residue. The rate of this transfer is sensitive to the binding of adenine nucleotides to the enzyme. The resulting N-nitrobenzofurazan ATPase has little or no activity. 2. The fluorescence of the N-nitrobenzofurazan group in the modified ATPase is quenched on binding of ADP. 3. Electrophoresis of the modified enzyme in sodium dodecyl sulphate on a 10% polyacrylamide gel shows that the fluorescence of the N-nitrobenzofurazan chromophore is exclusively in the beta subunit. 4. The rate of transfer of the nitrobenzofurazan group from tyrosyl oxygen to nitrogen on the enzyme is compared with the rate of transfer between model compounds. 5. The interaction of the N-nitrobenzofurazan ATPase with aurovertin is reported.
