Chemical, physical-chemical, and immunological properties of papain-solubilized human transplatation antigens.

Papain-solubilized HLA-A, -B, and -C antigens have been isolated from cadaveric spleens. The isolated material was homogeneous and comprised subunits with the apparent molecular weights 33,000 and 12,000. Amino acid analyses of a mixture of HLA antigen heavy chains obtained from a great number of spleens with different HLA antigen phenotypes revealed a composition that is very similar to that of individual HLA-A and -B antigens. Likewise, the NH2-terminal 30 residues of the HLA-antigen heavy chain mixture were virtually identical with that recorded for individual specificities. The circular dichroism spectra for the isolated HLA antigens and for free beta2-microglobulin revealed similarities with spectra recorded for immunoglobulin chains and domains. The HLA-antigen heavy chain may contain an appreciable amount of beta structure. Antibodies raised against free beta2-microglobulin react better with beta2-microglobulin in free form than when bound to the HLA-A, -B, and -C antigen heavy chains. This is due to the fact that free beta2-microglobulin can bind a maximum of four Fab fragments simultaneously, whereas the HLA-antigen-associated beta2-microglobulin can bind only two Fab fragments without dissociating from the heavy HLA-antigen subunit.