A New Serum Factor in Normal Rabbits

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Abstract
Evidence for the reaction of a naturally-occurring antibody (HR), previously shown to react specifically with Fab and Fab′ fragments of IgG, was extended to the demonstration of similar reactivity with Fab" fragments. Cross-adsorption and cross-inhibition experiments indicated that the reactions with Fab, Fab′ and Fab" fragments were different. Thus, it was concluded that normal rabbits possess at least three different specificities of HR. Moreover, the IgG molecule contains at least three different buried determinants, each exposed by papain, pepsin or CNBr digestion. A combination of sequential digestions of the fragments permitted the step-wise exposure of the determinants. From these efforts, it was possible to construct a modified scheme of that region of the heavy chain which is particularly susceptible to cleavage with papain, pepsin or CNBr, i.e., the hinge region. Furthermore, by taking into consideration the known amino acid sequence between the sites of papain and CNBr cleavage and the data presented, pertinent points of polypeptide chain folding were introduced as a means by which the determinants are concealed in the intact IgG molecule.

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