Characterization of Ileal Vitamin B12 Binding Using Homogeneous Human and Hog Intrinsic Factors

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Abstract
Elucidation of the mechanism of intrinsic factor (IF)-mediated vitamin B12 (B12) binding to ileal binding sites has been hampered by the use of crude or only partially purified preparations of IF in previous studies. We have used homogeneous human IF and hog IF isolated by affinity chromatography to study [57Co]B12 binding to ileal mucosal homogenates. The following observations were made: (a) Human IF-B12 and hog IF-B12 were bound to human, monkey, hog, dog, rabbit, mouse, hamster, and guinea pig ileal, but not jejunal, homogenates in amounts significantly greater than free B12 or B12 bound to five other homogeneous B12-binding proteins; (b) only IF-mediated B12 binding was localized to ileal homogenates and was inhibited by EDTA; (c) values for the association constant (Ka) for the various ileal homogenates mentioned above and human IF-B12 and hog IF-B12 ranged from 0.3 × 109 M-1 to 13.0 × 109 M-1. Apparent differences in the Ka for human IF-B12 and hog IF-B12 existed in most species; (d) the number of ileal IF-B12 binding sites per gram (wet weight) of ileal mucosa ranged from 0.3 × 1012 to 4.9 × 1012. The same value was always obtained with human IF-B12 and hog IF-B12 for any given homogenate preparation; (c) 100-fold excesses of free B12 or human IF and hog IF devoid of B12 did not significantly inhibit human IF-B12 and hog IF-B12 binding to human and hog ileal homogenates. These experiments performed with homogeneous IF indicate that: (a) gastric factors other than IF are not required for B12 binding to ileal IF-B12-binding sites: (b) the mechanism of ileal IF-B12 binding is different from that of free B12 or of B12 bound to non-IF-B12-binding proteins; (c) human IF and hog IF have different structures; (d) human IF-B12 and hog IF-B12 bind to the same ileal binding sites; and (c) human and hog ileal IF-B12 binding sites bind free B12 and human and hog IF devoid of B12 poorly, if at all.