Protein purification by anion exchange chromatography

1. A method for the purification of a divalent cation binding protein comprising the steps of:

(a) loading a first anion exchange resin material with the divalent cation binding protein in a loading buffer in the absence of divalent cations or at a low concentration thereof;

(b) eluting the divalent cation binding protein with an eluant comprising a counter-anion to form an eluate containing the divalent cation binding protein;

(c) supplementing the eluate obtained in step (b) with at least one divalent cation and increasing the pH;

(d) loading a second anion exchange resin material with the supplemented eluate obtained in step (c); and

(e) collecting the flow-through from step d) containing the divalent cation binding protein, wherein the divalent cation binding protein is at least 95% w/w pure with respect to host cell protein impurities.

2. The method according to claim 1 , wherein in step (c) the pH of the supplemented eluate is increased by at least 0.5 pH units.

3. The method according to claim 1 , wherein the eluant in step (b) has a conductivity that is higher than the conductivity of the loading buffer in step (a), and wherein the supplemented eluate in step (c) has a conductivity that is lower than the conductivity of the eluant in step (b).

4. The method according to claim 1 , wherein the at least one divalent cation in step (c) is selected from the group consisting of Ca 2+ , Be 2+ , Ba 2+ , Mg 2+ , Mn 2+ , Sr 2+ , Zn 2+ , Co 2+ , Ni 2+ , and Cu 2+ , or combinations thereof.

5. The method according to claim 1 , wherein the first and second anion exchange resin materials each have a positively charged group which is independently selected from the group, consisting of diethylaminoethane (DEAE), dimethylaminoethane (DMAE), trimethylaminoethyl (TMAE), polyethyleneimine (PEI), quaternary aminoalkyl, quaternary aminoethane (QAE), and quaternary ammonium (Q).

6. The method according to claim 1 , wherein the first and second anion exchange resin materials each carry a primary amine as ligand which is independently selected from the group consisting of aminohexyl, benzamidine, lysine, and arginine.

7. The method according to claim 1 , wherein the divalent cation binding protein is a calcium binding protein.

8. The method according to claim 1 , wherein the divalent cation binding protein is a vitamin K-dependent protein.

9. The method according to claim 1 , wherein the divalent cation binding protein is selected from the group consisting of Factor II, Factor VII, Factor IX, Factor X, Protein C, Protein S, Annexin and calmodulin.

10. The method according to claim 1 , wherein the pH of the loading buffer in the loading step (a) is ≦pH 7.4.

11. A method according to claim 1 , wherein the method further comprises a step of washing the loaded anion exchange resin material with a washing buffer in the absence of divalent cations after the loading step (a).

12. A method according to claim 11 , wherein the eluant in step (b) has a conductivity that is higher than the conductivity of the washing buffer used after the loading step (a).

13. A method according to claim 11 , wherein the washing buffer has a pH≦pH 7.4.

14. The method according to claim 11 , wherein the pH of the washing buffer is in a range selected from the group consisting of: between pH 5.5 and pH 7.0, between pH 6.0 and pH 7.0, and between pH 6.5 and pH 7.0.

15. The method according to claim 1 , wherein the divalent cation binding protein is selected from the group consisting of Factor IX, Factor VII, and Annexin V.

16. The method according to claim 1 , wherein the pH of the loading buffer in the loading step (a) is in a range selected from the group consisting of: between pH 5.5 and pH 7.0, between pH 6.0 and pH 7.0, and between pH 6.5 and pH 7.0.

17. The method according to claim 1 , wherein the second anion exchange reaction results in a CHO reduction factor of >27.