| Abstract: | In newborn rats, passive immunity is acquired from the mother by selective transport across the gut wall of immunoglobulin (IgG) present in colostrum and milk. Ultrastructural and physiologic studies of this mechanism have shown that the binding and uptake of IgG exhibits saturation kinetics and stereochemical specificity consistent with it being a receptor-mediated process. We report here the isolation and purification of a protein from membranes of neonatal rat enterocytes that binds immunoglobulins. The basis of our purification procedure is the extraction of this IgG-binding protein from isolated membranes in the absence of detergents and its biospecific elution from an IgG affinity column. This purified protein consists of two similar polypeptides of 52,000 and 48,000 Mr. The interaction of this purified protein with immunoglobulin is isotype dependent, with specificity for IgG and its Fc fragment, and pH dependent, with optimal binding at the intraluminal pH of 6.0. This intestinal IgG-binding protein is found in enterocytes of the proximal intestine during the early postnatal period, but is absent after weaning when transport of IgG ceases. Our results suggest that this purified intestinal IgG-binding protein functions in the transepithelial transport of IgG in rat neonates. |
