At least one but possibly two Na⁺ ions bind to the apo form of ASCT2. [Na⁺] dependence of the leak anion current (normalized to the response at 140 mM Na⁺). The solid line represents a fit to the equation I = A [Na⁺]/(K₁ + [Na⁺]) + B [Na⁺]/(K₂ + [Na⁺]). Here, A and B are scaling parameters, and the values for K₁ and K₂ were 0.25 ± 0.06 mM and 51 ± 10 mM, respectively. The intracellular solution contained 120 mM NaSCN and 10 mM alanine. The extracellular solution contained 140 mM NaMes (V = 0 mV).