Figure 4.

Ca²⁺ affinity of ΔpH-dependent Ca²⁺ transport. Vesicles were isolated from mutant K616 transformed with vector. Left, Uptake was determined at 40 s after 3 mm ATP was added to a reaction medium containing 10 nm to 3 μm free Ca²⁺. ΔpH-dependent Ca²⁺ transport (ΔpH, •) was determined as activity that was inhibited by 5 μm gramicidin and 0.5 μm bafilomycin. Vanadate-sensitive Ca²⁺ transport refers to activity inhibited by 200 μm sodium vanadate (ΔVan, ○). Right, Lineweaver-Burk plot. ΔpH-dependent uptake has a K_m for Ca²⁺ of 350 ± 50 nm (n = 5).