Identification and characterization of intrinsic factor and cobalophilin from pig ileal and pyloric mucosa

Pig ileal mucosa was found to bind about 240 ng vitamin B12/g and to contain two vitamin B12-binding proteins. One was highly active in the Schilling test, behaved immunologically as intrinsic factor and was responsible for about half of the total vitamin B12-binding capacity. The other binder was identified as cobalophilin (R-protein). Immunochemical purification of these proteins from pig ileum and pylorus was performed and the molecular characteristics (sedimentation and diffusion coefficients, Stokes radii, frictional ratios and molecular weights) of their vitamin B12 complexes were estimated. Isoelectric focusing revealed differences between the ileal and pyloric intrinsic factors but not between the cobalophilins. The mean isoelectric points of the pyloric and ileal intrinsic factors were pH 5979 and 5.30, respectively, whereas the corresponding figures for the cobalophilins were 4.13 and 4.10.