FIGURE 3.

[³H]Melibiose efflux and exchange. Intracellular [³H]melibiose was expressed as a percentage of the zero time point in the presence of external 200 mm Na⁺ or Li⁺. Open (Efflux) and filled (exchange) symbols represent equimolar cation concentrations across the membrane. The dashed line represents external 200 mm Na⁺ or Li⁺. A, de-energized RSO vesicles were pre-equilibrated with 20 mm [³H]melibiose without Na⁺ and Li⁺. Right-filled hexagons and stars represent the presence of external Na⁺ and Li⁺, respectively. The dotted line represents the presence of 20 mm external cations. RSO vesicles were also treated with 3.6 mm MIANS for 5 h at 23 °C and diluted into 100 mm potassium P_i, pH 7.5, in the absence of melibiose (crossed square). B and C, de-energized RSO vesicles were pre-equilibrated with 20 mm [³H]melibiose and 20 mm Na⁺ (B) or Li⁺ (C). The left-filled symbol represents the dilution buffer without Na⁺ or Li⁺ (20/0 mm). The right-filled symbol represents the dilution buffer containing 200 mm Na⁺ or Li⁺ (20/200 mm).