Releasing Factor and Endogenous Vitamin B12

Intestinal Mucosa and Vitamin B₁₂ Absorption

“…[43] then described the localization of this factor in the proximal segment of the small intestine. However, their final conclusion, based on the results of later experiments, was that the observed release of B,., from IF was effected by an exchange with free B12 [46]. Donaldson et al [18] also demonstrated easy exchange between IF-bound and free cyanocobalamin.…”

TRANSIENT (REVERSIBLE) MALABSORPTION OF VITAMIN B12

“…It would thus appear that there may be functional inability of the ileum to absorb vitamin B,, via the intrinsicfactor-dependent mechanism, despite apparent structural integrity. Whether such malabsorption results &om lack of 'releasing factor' (Highley, Streiff, Ellenbogen, Herbert and Castle, 1965), lack of receptors (Fig. I), or some other factor is unknown at present.…”

Acquired Selective Vitamin B12 Malabsorption

“…However, more recent experiments (Highley et al 1965) have challenged this theory, since it might be a question of a repIacement of the radioactively labelled intrinsic factor-bound vitamin BIZ by endogenous vitamin B1, from the small intestine, whereby the labelled vitamin B12 becomes dialyzable. Nevertheless, experiments carried out by Colle et al (1961), Movitt et al (1963), Resnick et al (1963, and the author of the present paper (Hippe 1966) have proved the existence of factor(s) in the intestinal juice, important for the absorpotin of vitamin B12, as is the case in the patient described here.…”