A derivative of R-type cyanocobalamin binding proteins in the human intestine.  A candidate antibacterial molecule. 
Human jejunal fluid contains a protein which has a molecular radius of 3.37 nm, an Mr or 60600 and a mean pI of 3.35 and binds cobalamin with a K of 0.1 .  10(9) l/mol.  This protein also couples cobalamin analogues lacking the nucleotide moiety, cross-reacts with R-type protein and is resistant to proteolysis in the intestine.  These findings refute the hypothesis that cobalamin-analogue binders are present and effectively inhibit the bacterial uptake of analogues in the intestine.