Recombinant human intrinsic factor expressed in plants is suitable for use in measurement of vitamin B12.

Abstract

Competitive binding approaches with use of specific binding proteins are the most commonly used methods to measure vitamin B12 in laboratory medicine. Various binding proteins have been used in these methods, including intrinsic factor (IF), pooled human or chicken serum, transcobalamin (TC), and saliva (1)(2). The most widely used of these are non-human IF preparations, usually obtained from hog gastric mucosa. Numerous problems have been reported with their application, however, including difficulty of purification, instability on storage, and variation in the binding capacity connected with changes in serum protein and vitamin B12 concentrations (1)(2). If IF is not highly purified, it may contain haptocorrins (also called R proteins), which bind not only vitamin B12 but also related metabolically inactive compounds that may be present in the sample, thereby causing artificially increased vitamin B12 results (1)(3). To circumvent the problems associated with nonhuman IF, we recently expressed human IF in plants and obtained a product free of endogenous vitamin B12 and contaminating vitamin B12-binding proteins (4). In the current study, we examined the feasibility of using this recombinant human IF for measurement of vitamin B12 bound to TC. Human IF was expressed in the recombinant plant Arabidopsis thaliana and purified as described previously (4). As a first step, recombinant human IF was coupled to magnetic beads (Dynabeads, M-280 Tosylactivated; DYNAL) according to the protocol for ligand-binding …