Intrinsic factor, free of R proteins, can be prepared from mouse stomach and used in a ligand assay specific for "true" cobalamin.

A ligand assay specific for cobalamin that uses mouse stomach as the source of intrinsic factor has been developed. When mouse stomach extract incubated with radiocobalamin is fractionated by gel chromatography, the radioactive complex elutes as a single peak with apparent molecular weight of 54,900. Formation of the complex is greater than 98% inhibited by human anti-intrinsic factor antibody. When the equivalent of 10,000 pg/ml of cobinamide is added to serum, the apparent cobalamin concentration detected averages 8.5 pg/ml. Correlation with the Lactobacillus leichmannii microbiologic assay results in the regression equation y = 0.97x + 20. In six patients who had megaloblastic anemia the serum cobalamin by the mouse intrinsic factor ligand assay ranged from 0 to 9 pg/ml. Because the primary source of intrinsic factor is free of R proteins, there is no need for extensive purification of the extract. The assay is sensitive, precise, and accurate, and no more difficult to perform than other conventional ligand assay procedures.