Measurement of peptidoglycan antibodies by a radioimmunoassay

Staphylococcus epidermidis peptidoglycans solubilized by sonication or lysozyme digestion, and synthetic peptidoglycan analogs such as HSA-carboxymethyl-Gly-L-Ala-L-Ala-D-Ala-D-Ala (HSA-pentapeptide) or L-Ala-gamma-D-Glu-L-Lys-D-Ala-D-Ala (pentapeptide) have been labeled with 125I and tested for their applicability in the radioactive antigen binding assay. Use of radioiodinated Staph. epidermidis peptidoglycans was found to be considerably impeded by the presence of at least 2 different antigenic sites on such molecules, the pentapeptide and the glycan determinant. Application of labeled HSA-pentapeptide was limited by the necessity to use PEG for precipitation of Ag-Ab-complexes and by short linear potions of binding curves. However, the synthetic pentapeptide hapten, radioiodinated by the active ester method of BOLTON and HUNTER, proved to be a most useful regent for the selective measurement of pentapeptide antibody. Inhibition studies indicated that the immunological specificity of the labeled hapten was retained. Pentapeptide binding curves were linear from 15-500 g/ml of antibody. Generally, there was good agreement between pentapeptide antibody concentrations measured by radioimmunoassay and quantitative precipitation.