GFL binding to heparin. (A) Elution of ¹²⁵I-GFLs from a heparin–Sepharose column by a step-wise NaCl gradient. PSPN elutes at a 0.5-M NaCl concentration, indicating a very weak interaction with heparin. GDNF elutes from the heparin column at a 0.75–1-M NaCl concentration, ARTN elutes at a 1–1.5-M NaCl concentration, and NRTN elutes at a 1.5-M NaCl concentration. (B) Heparin binding to immobilized GFLs was followed by SPR using the IAsys optical biosensor. The K_ds were determined from dynamics as well as equilibrium data (insets), and their values are indicated in the graphs. Error bars show SEM from three to four experiments for each condition. K_on is the association rate constant. arcsec, arcsecond.