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Intrinsic factor (IF) is the gastric protein that promotes the intestinal uptake of vitamin B12 . Gastric IF from animal sources is used in diagnostic tests and in vitamin pills. However, administration of animal IF to humans becomes disadvantageous because of possible pathogenic transmis-sion and contamination by other B12binders. We tested the use of recombinant plants for large-scale production of pathogen-free human recombinant IF. Human IF was successfully expressed in the recombinant plantArabidopsis thaliana | Eur. J. Biochem. 270 3362-3367 2003 FEBS 2003 doi 10.1046 j.1432-1033.2003.03716.x Human intrinsic factor expressed in the plant Arabidopsis thaliana Sergey N. Fedosov1 Niels B. Laursen1 2 Ebba Nexo3 Soren K. Moestrup4 Torben E. Petersen1 Erik 0. Jensen5 and Lars Berglund1 2 1Protein Chemistry Laboratory Department of Molecular and Structural Biology University of Aarhus Denmark 2Cobento Biotech A S Science Park Aarhus C Denmark 3Department of Clinical Biochemistry AKH Aarhus University Hospital Denmark 4Department of Medical Biochemistry University of Aarhus Denmark 5Laboratory of Gene Expression Department of Molecular and Structural Biology University of Aarhus Denmark Intrinsic factor IF is the gastric protein that promotes the intestinal uptake of vitamin B12. Gastric IF from animal sources is used in diagnostic tests and in vitamin pills. However administration of animal IF to humans becomes disadvantageous because of possible pathogenic transmission and contamination by other B12 binders. We tested the use of recombinant plants for large-scale production of pathogen-free human recombinant IF. Human IF was successfully expressed in the recombinant plant Arabidopsis thaliana. Extract from fresh plants possessed high B12-binding capacity corresponding to 70 mg IF per 1 kg wet weight. The dried plants still retained 60 of the IF activity. The purified IF preparation consisted of a 50-kDa glycosylated protein with the N-terminal sequence of mature IF. Approximately one-third of the protein was cleaved at the internal site . PSNPflGPGP. The key properties of the preparation obtained were identical to those of native IF the binding curves of vitamin B12 to recombinant IF and gastric IF were the same as were those for a B12 analogue cobina-mide which binds to IF with low affinity. The absorbance spectra of the vitamin bound to recombinant IF and gastric IF were alike as was the interaction of recombinant and native IF with the specific receptor cubilin. The data .