Figure 5.

Rates of positional isotope exchange or ATP hydrolysis. (A) P-gp supersomes. The fractions of the exchanging species γ-¹⁸O₄-ATP in red, γ-¹⁸O₃-ATP in blue, and γ-¹⁸O₂-ATP in green as a function of the incubation time are shown. Also, the time dependence of the ratio γ-¹⁸O₃-ATP/γ-¹⁸O₄-ATP is presented (black line). (B) Purified liposome-reconstituted P-gp. The hydrolysis rate (red line) and the time dependence of the ratio γ-¹⁸O₃-ATP/γ-¹⁸O₄-ATP (black line) are shown. The reactions were conducted as described in the text, and ³¹P NMR spectra were recorded at the times indicated. ATP concentrations and the PIX rates were determined from the areas of the ³¹P NMR signals of the γ-phosphorus atom of ATP corresponding to the γ-¹⁸O₄-ATP, γ-¹⁸O₃-ATP, and γ-¹⁸O₂-ATP moieties.