Fig. 4. Saturation kinetics of cystinosin-mediated cystine uptake. (A) Plot of the velocity (V) of [³⁵S]cystine uptake versus increasing substrate concentration (S) for mock-transfected (white squares) and cystinosin-ΔGYDQL-expressing (black squares) cells over an 8 min uptake period (linear phase). A deduction of background levels demonstrates that cystine uptake by cystinosin-ΔGYDQL is saturable (black triangles). (B) A linear Eadie–Hostee plot of the cystinosin-ΔGYDQL-dependent data demonstrates that cystine transport follows Michaelis–Menten kinetics. K_M = 350 µM and V_max = 507 pmol/min per well for this experiment.