Binding of exogenous protein fragments to native proteins: possible explanation for the overestimation of uptake of extrinsically labelled macromolecules from the gut.
Investigators studying the transport of macromolecules across mucosal surfaces have noted that techniques involving exogenously labelled macromolecules result in estimation of transport which exceed estimates based on immunochemical methods. The present study tested the possibility that radio-labelled fragments of macromolecules might bind to host proteins resulting in an apparent increase in 'size' of the fragments. Fragments obtained by dialysis of bovine serum albumin (BSA) labelled with tritium gas by Wilzbach method, bound to normal rabbit serum in vitro and in vivo. Binding to BSA exceeded binding to bovine gamma globulin (BGG). Dinitrophenylated BGG was administered by gavage to adult rats; DNP-bearing fragments were identified in their serum and recovered by gel filtration. After radio-labelling, these serum fractions also bound to macromolecular constituents of normal rat serum. These findings suggest that several different fragments of proteins may bind to native serum components. These results may bear upon in vivo observations since it is likely that following intestinal instillation of extrinsically labelled protein, labelled amino acids or small peptides are generated by the digestive process, become absorbed, bind to serum macromolecules and thereby mimic the uptake of macromolecules.