Ab Fragments Research Articles

Segmental flexibility in an antibody molecule

The rotational motions of an antibody molecule were investigated by nanosecond fluorescence polarization techniques. The fluorescent chromophore, ϵ-dansyl- l-lysine, was specifically bound to the combining sites of antidansyl antibody. The emission anisotropy of the bound dansyl group was measured as an explicit function of time following excitation by a nanosecond light pulse. The F( (ab′)2) and F ab fragments of the antibody were studied in the same way. The striking finding is that the antibody molecule displays a discrete mode of flexibility in the nanosecond time range. The F ab portions of the intact antibody molecule are free to rotate over an angular range of the order of 33 degrees in times of nanoseconds. The rotational correlation time for this restricted segmental motion is 33 nsec. The over-all rotational motion of the antibody molecule has a correlation time of 168 nsec, indicating that the antibody has a markedly non-spherical shape. Our findings are consistent with an important feature of the model proposed by Valentine & Green (1967), namely that there is a flexible joint at the junction of the F ab segments. This joint may be biologically significant in facilitating the formation of antibody-antigen complexes. The facile segmental flexibility exhibited by the antibody exemplifies a structural design which is probably used in other large proteins and in organized macromolecular assemblies.

The Biologic Activities of Guinea Pig Antibodies

Abstract Guinea pig immunoglobulins have been separated into the γ1 and γ2 classes by chromatography on DEAE cellulose. Both types show charge heterogeneity. The heterogeneity of the γ2 immunoglobulins is attributed to the F(ab) fragments. As noted previously by others, the ability to mediate passive cutaneous anaphylaxis (PCA) in the guinea pig skin is confined almost entirely to the γ1 immunoglobulins. Both types of immunoglobulins participate in C-dependent allergic reactions of the immediate type. These include the generation of anaphylatoxin, the mediation of reverse passive Arthus reactions, as well as allergic histamine release from normal rabbit platelets.

In Vivo and in Vitro Properties of Intact and Pepsin-Digested Heterologous Anti-Mouse Thymus Antibodies

Summary Rabbit anti-mouse thymus globulin (AMT-IgG) had a strong inhibitory effect of IgM antibody-forming spleen cells of mice when injected prior to the antigen. No effect was observed when AMT-IgG was injected after or simultaneously with the immunizing dose of sheep red cells. Pepsin-digested bivalent antibody did not inhibit formation of antibody-synthesizing spleen cells nor did it prolong survival of allogeneic skin grafts. AMT-IgG as well as the bivalent fragment F(ab')2 stimulated uridine incorporation of thymocytes in vitro. The F(ab) fragment, however, had no stimulatory effect though it protected the cells when exposed to complete cytotoxic antibodies in the presence of complement. The possible mechanism of immediate immunosuppression by which AMT-IgG acts in vivo is discussed. Biologic properties of the Fc fragment are implicated in this action.

Studies on Rabbit Lymphocytes in Vitro

Summary The peripheral lymphocytes of rabbits may be stimulated in vitro to undergo blast transformation by the F(ab′)2 and F ab fragments of sheep globulin containing antibody activity to rabbit IgG. This indicates that the secondary biologic properties of immunoglobulin molecules and the capacity to precipitate with antigen are not necessary for blastogenic activity. The initial event in the induction of blast transformation by specific antiserum and by specific antigen may be the same mechanism, the primary union of antigen and antibody.