Combining Sites of IgG and IgM Antibodies of Poly‐d‐Alanyl Specificity

Goats and rabbits were immunized with poly‐d‐alanyl ribonuclease. Anti‐poly‐d‐alanyl antibodies were isolated from the goat and rabbit sera by means of a water‐insoluble immuno‐adsorbent prepared from poly‐d‐alanyl rabbit serum albumin and bromoacetyl cellulose. The isolated antibodies and sera were separated into IgM and IgG fractions. The ability of the antibodies and of the serum fractions to precipitate with poly‐d‐alanyl human serum albumin and to inactivate poly‐d‐alanyl bacteriophage T4 was inhibited with alanine peptides of increasing size. In all cases the efficiency of inhibition as a function of the size of the inhibitory peptide increased up to the tetrapeptide. The penta‐d‐alanine, on the other hand, showed an extent of inhibition similar to the tetrapeptide. These results suggest that the antigen‐combining sites of both IgG and IgM antibodies are similar in size and capable of accomodating a peptide composed of four alanine residues.