AJP - Gastrointestinal and Liver Physiology, Vol 245, Issue 1 72-G77, Copyright © 1983 by American Physiological Society
Effect of bile and bile acids on binding of intrinsic factor to cobalamin and intrinsic factor-cobalamin complex to ileal receptor
B. Seetharam, M. Jimenez and D. H. Alpers
Human bile or commercially available bile acids bind [57Co]cobalamin (Cbl)
due to the presence of "R"-type binders, which are normally present in the
former and present as a contaminant in the latter. The competition of these
R proteins for the binding of Cbl by intrinsic factor (IF) could explain
the in vivo inhibition attributed previously to the bile acids themselves.
R protein seems to be involved in the inhibition of Cbl binding because
protease digestion of either bile or bile acid abolishes the Cbl binding
ability. Moreover, antibody to R protein abolishes the inhibition. Bile or
bile acids do not have a direct effect on either purified IF or the IF-Cbl
receptor molecule, even though bile acids increase the attachment of
IF-[57Co]Cbl to ileal brush-border membranes. These data demonstrate two
steps where components of bile could affect Cbl absorption: the binding of
Cbl to IF and of IF-Cbl to its ileal receptor. It is not clear whether
these in vitro phenomena are important for the normal absorption of Cbl in
vivo.
