Two Simultaneous Binding Sites for Nucleotide Analogs Are Kinetically Distinguishable on the Sarcoplasmic Reticulum Ca²⁺-ATPase

Erythrosin B and eosin Y stimulate p-nitrophenyl phosphate hydrolysis by purified sarcoplasmic reticulum Ca(2+)-ATPase by nearly 2-3 fold in the presence of Ca(2+). This stimulation is not due to the change on the apparent affinity for substrate but is indeed due to acceleration of the turnover rate of the enzyme. Stimulation reaches a maximum at approximately 5 microM erythrosin or 20 microM eosin and is strictly dependent on the presence of Ca(2+) in reaction media, while higher concentrations of dye progressively inhibit phosphatase activity. Labeling with fluorescein isothiocyanate (FITC) largely shifts the Km for p-nitrophenyl phosphate (pNPP) and completely abolishes the stimulation of phosphatase activity induced by erythrosin in the presence of Ca(2+), apparently by FITC impairing dye binding to an activator site and allowing only manifestation of an inhibitory binding site. In the absence of Ca(2+), both erythrosin and eosin inhibit pNPP hyrolysis with Ic50 values 3-4 fold higher than the maximally stimulatory enzyme with FITC, which by its turn does not affect pNPPase activity in absence of Ca(2+). It is suggested that stimulation and inhibition of phosphatase activity are related to two simultaneous and physically different nucleotide analog binding sites.