For the specific activity experiment, GAD was purified by nickel ligand-based his-tag affinity chromatography using HisTrap HP (GE Healthcare, Uppsala, Sweden) and a chromatography system (AKTAexplorer 100Air; GE Healthcare, Uppsala, Sweden) with the following method: 5 column volume (CV) equilibration, sample loading, 10 CV primary washing with buffer A, 10 CV secondary washing by step elution at 5% buffer B, 10 CV step elution at 100% buffer B and re-equilibration.