Abstract
Interaction of 59Fe-labeled rat transferrin, human lactoferrin, and bovine lactoferrin with rat small intestinal brush-border membrane vesicles was investigated with the use of a rapid filtration technique. Specific binding of 59Fe-labeled rat transferrin and bovine lactoferrin to brush-border membrane vesicles from suckling and adult rats was identified. In contrast, no binding of human lactoferrin occurred. The presence of transferrin receptors on the brush-border membrane of suckling rats was confirmed by immunoblotting, and the molecular mass of the receptor was 96 kDa under nonreducing conditions. Scatchard plot analysis indicated 2.4 x 10(14) binding sites/mg of membrane protein with an affinity constant (Ka) of 4.9 x 10(6) M-1 for rat milk transferrin and 2.2 x 10(14) binding sites/mg of membrane protein with a Ka of 3.2 x 10(6) M-1 for bovine lactoferrin. Bovine lactoferrin competitively inhibited the binding of rat transferrin to the brush-border membrane vesicles. Deglycosylation of rat transferrin and bovine lactoferrin had no influence on the binding of these proteins. The results suggested that bovine lactoferrin bound to the receptor for rat transferrin on the brush-border membrane and that the polypeptide chain rather than the glycan moiety is responsible for the interaction of these proteins with the rat brush-border membrane.
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