Papin-solubilized Ag-B antigens. II. Characterization of small sized Ag-B molecules.

Papain solubilization of rat Ag-B histocompatibility antigens produces Ag-B molecules of about 59,000 daltons which have been shown to contain two fragments bound noncovalently: one fragment about 37,000 daltons carrying Ag-B allospecificity, and another about 11,000 daltons, an apparent rat homologue of human beta2-microglobulin. Beside the 59,000-dalton Ag-B molecules, papain digests of liver cell membranes of ACI strain rats were found to contain Ag-B molecules of about 25,000 and 35,000 daltons. These smaller Ag-B molecules carried Ag-B private specificity of the rat strain (i.e., Ag-B4), as did the 59,000-dalton Ag-B molecules, and accounted for 40% of the solubilized Ag-B alloantigenic activity. The smaller Ag-B molecules were tested for the antigenic specificities that are characteristic of each of the two fragments of the 59,000-dalton molecules and detected by rabbit antiserum against rat cell membranes. The 35,000-dalton Ag-B molecules were found to contain the Ag-B 11,000-dalton fragment (i.e., rat beta2-microglobulin homologue) and to differ from the 59,000-dalton Ag-B molecules only in absence of a part of the 37,000-dalton fragment portion. The 25,000-dalton Ag-B molecules did not contain the rat beta2-microglobulin homologue and contained only a single component that is similar to the alloantigenic fragment portion of the 35,000-dalton Ag-B molecules. Similar 25,000-dalton Ag-B molecules (carrying Ag-B1 private specificity) of a single component were found in Fischer rat material. They accounted for 10% of the solubilized Ag-B alloantigenic activity.