Vitamin B12-binders in human body fluids. I. Antigenic and physico-chemical characteristics.

Three factors binding vitamin B are detected in human gastric juice by autoradiographs of immunoelectrophoresis. Binder 1 is present only in gastric juice and gastric mucosa extract and corresponds to intrinsic factor. Binder 2 is present also in other excretions such as bile, lactoserum, saliva, in serum and leucocyte extracts. Binder 3 present in saliva and gastric juice in lower quantity than binder 2, seems derived from this last one by attachment of sialic acid molecules. There is no immunological identity between intrinsic factor and the other binders without intrinsic factor activity. Binder 3 is resistant to proteolysis and . Binder 2 is modified by peptic digestion but not . Binder 1 is insensitive to proteolysis if saturated by vitamin B, but loses its binding capacity and/or immunological properties if the digestion takes place before fixation of vitamin B. The three binders seem to have the same affinity for vitamin B as judged by dialysis experiments, and the same molecular weight of 60,000 as judged by volume exclusion on Sephadex G-200.