The plasma membrane Ca²⁺ pump mutant lysine⁵⁹¹ → arginine retains some activity, but is still inactivated by fluorescein isothiocyanate

Inactivation of the wild-type human plasma membrane Ca²⁺ pump (isoform 4b) by fluorescein isothiocyanate is accompanied by covalent modification of Lys⁵⁹¹. The mutation of Lys⁵⁹¹ to arginine reduced the Ca²⁺ transport activity to 35% of the wild-type, and diminished the amount of acylphosphate formed from ATP by a corresponding amount. When this mutant was treated with fluorescein isothiocyanate, the enzyme was still irreversibly inactivated, even though no reactive residue was available at position 591. The results show that, although Ca²⁺ pump function is sensitive to the residue at position 591, Lys⁵⁹¹ is not essential for enzyme activity. They also demonstrate that irreversible inhibition of the plasma membrane Ca²⁺ pump by fluorescein isothiocyanate does not require the covalent modification of Lys⁵⁹¹. This indicates that fluorescein isothiocyanate reacts with lysine residues at other positions in addition to Lys⁵⁹¹.