Abstract
Determinations of reaction stoichiometry demonstrate that the covalent incorporation of one molecule of 5''-isothiocyanatofluorescein can inactivate one molecule of sodium and potassium ion activated adenosinetriphosphatase in agreement with earlier determination of this stoichiometry. Several different modified peptides are produced, however, when the modified enzyem is digested with trypsin. One of these peptides has been identified as HLLVMK(thioreidylfluorescein)GAPER by use of a specific immunoadsorbent. The modified lysine is lysine 501 in the amino acid sequence of the .alpha. polypeptide of (Na+ + K+)-ATPase. This peptide has been previously isolated from such digests [Farley, R. A., Tran, C. M., Carilli, C.T., Hawke, D., and Shively, J. E. (1984) J. Biol. Chem. 259, 9532-9535]. The other specifically modified peptides have been purified and identified by amino acid sequencing. Their sequences identify lysing 480 and lysine 766 from the .alpha. polypeptide as amino acids modified by 5''-isothiocyanatofluorescein in reactions senstive to the addition of ATP and responsible for inactivation of the enzyme.

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