Purification of refolded RBP on HPLC to obtain holo-RBP 100% loaded with retinol. Nickel resin-purified refolded RBP is applied to ion exchange column AX-300 by a NaCl step gradient (220 mM 10 min, 360 mM 15 min, and 1,000 mM 15 min). The correctly folded holo-His-RBP is released and eluted at 360 nM NaCl. Absorption spectra (250 nm-400 nm) of the peaks corresponding to correctly folded holo-RBP, misfolded RBP-1, misfolded RBP-2, and contamination are also shown (protein peak is indicated by a blue vertical line and retinol peak is indicated by red vertical line).