AJP - Gastrointestinal and Liver Physiology, Vol 239, Issue 6 452-G456, Copyright © 1980 by American Physiological Society
Divalent cations and vitamin B12 uptake by intestinal brush-border membrane vesicles
R. C. Beesley and C. D. Bacheller
Brush-border membrane vesicles from hamster intestine were employed to
investigate uptake (binding) of vitamin B12 (B12). Ileal vesicles took up
25 times more B12 than did jejunal vesicles. Uptake of B12 by ileal
vesicles was dependent on intrinsic factor (IF) and required Ca2+.
Increasing the Ca2+ concentration caused an increase in uptake of B12
reaching a maximum at approximately 8 mM Ca2+. At high Ca2+ concentrations,
6-8 mM, Mg2+ had little effect on uptake of B12. At low Ca2+
concentrations, up to 2 mM, Mg2+ stimulated B12 uptake. Mg2+, Mn2+, and, to
a lesser extent, Sr2+ stimulated Ca2+-dependent B12 uptake, but Zn2+, Ba2+,
Na+, K+, and La3+ did not. B12 was apparently not metabolized and was bound
as IF-B12 complex, which could be removed with
(ethylenedinitrilo)tetraacetic acid (EDTA). Our results suggest that two
types of divalent cation reactive sites are involved in binding of IF-B12.
One is Ca2+ specific. The other is less specific reacting with Mg2+, Mn2+,
Sr2+, and perhaps Ca2+ itself, thereby stimulating Ca2+-dependent binding
of IF-B12 to its ileal receptor.
