Figure 4.

Concentration-dependent uptake of [¹⁴C]urate in multidrug resistance protein (MRP) 4 membrane vesicles. The uptake rates of urate by MRP4 and human embryonic kidney (HEK)-EYFP control vesicles were measured at different concentrations of [¹⁴C]urate. Net ATP-dependent transport was calculated by subtracting background levels in the absence of ATP (substituted by AMP). MRP4-dependent transport was determined by subtracting control transport, and non-linear regression analysis was performed. Data were described significantly better according to a Hill equation with a slope factor (n) of 2.1 (solid line) as compared with a model with n=1 (dotted line), indicating positive cooperativity (P<0.005, F-test). Values are shown as mean and s.e.mean of three different experiments performed each in triplicate.