Incorporation of 4.5 nmol fluorescein isothiocyanate/mg rabbit sarcoplasmic reticulum, or of 7.4 nmol/mg purified ATPase, was sufficient to inhibit the activity completely. These results are not consistent with the suggestion (Pick, U. and Karlish, S.J.D. (1980) Biochim. Biophys. Acta 626, 255–261) that 2 mol ATPase were inhibited by each mole of reagent incorporated. A single labelled peptide was purified from the inhibited ATPase and it was shown that Lys 3/190, 10 residues from the N-terminus of tryptic fragment B, was the reactive lysine residue. This site is close to a potential nucleotide-binding fold in the ATPase sequence. A similar peptide showing only 2 conservative replacements was isolated from the sarcoplasmic reticulum of the lobster.
