Isolation and Characterization of an Abnormal Human Intrinsic Factor

A patient has been described previously who presented at age 13 with vitamin B₁₂ (B₁₂) deficiency secondary to a functionally abnormal intrinsic factor (IF). IF has now been isolated from the gastric juice of the patient, his sister, and both parents, who are first cousins, by using affinity chromatography on B₁₂-Sepharose. Patient IF appeared normal in terms of (a) B₁₂ binding, (b) mol wt, (c) total amino acid and carbohydrate composition, and (d) immunodiffusion with rabbit anti-patient and anti-normal IF sera. After adsorption with normal IF, however, anti-patient IF serum precipitated the various IFs as follows: patient IF (> 95%); mother, father, and sister IF (50%); and normal IF (< 10%). Additional adsorption with mother, father, or sister IF completely inhibited the precipitation of patient IF. The association constant determined for patient IF-B₁₂ and human ileal mucosal homogenates (0.1 × 10⁹ M^-1) was 60-fold lower than that determined with normal IF-B₁₂ (6.0 × 10⁹ M^-1). Intermediate amounts of ileal IF-B₁₂ binding were observed with mother, father, and sister IF-B₁₂. These in vitro studies were supported by multiple Schilling tests, performed with a totally gastrectomized volunteer, that gave the following mean urinary excretions of [⁵⁷Co]B₁₂: free B₁₂ (0.5%); + patient gastric juice (2.6%); + mother or father gastric juice (17%); and + normal gastric juice (26%). These studies demonstrate that the patient is homozygous and that the mother, father, and sister are heterozygous for a structurally abnormal IF that has a markedly decreased, but not absent, affinity for ileal IF-B₁₂ receptors. These studies also indicate that the B₁₂ and ileal binding sites are located on different portions of the IF molecule.

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